Feat: Replace Load More with Previous/Next pagination in library

Page size changes from 50 to 24. Library now shows discrete page navigation
with a "Page N of M" indicator, total image count, and URL state (?page=N)
so pages are bookmarkable and the browser Back button works. Tag filter
resets to page 1. Out-of-range page params are clamped silently.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

specs/015-library-pagination/tasks.md

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+# Tasks: Library Pagination UI
+
+**Input**: Design documents from `specs/015-library-pagination/`
+**Prerequisites**: plan.md ✅, spec.md ✅, research.md ✅, contracts/pagination-query.md ✅, quickstart.md ✅
+
+**Tests**: Tests accompany each implementation task per §5.1. All changes are in `ui/src/app/library/library.component.ts` and its spec file.
+
+**Organization**: No setup or foundational phase needed — the Angular project and library component already exist. Phase 1 implements US1 (page navigation controls). Phase 2 adds US2 (URL state). Polish runs lint and manual verification.
+
+## Format: `[ID] [P?] [Story] Description`
+
+- **[P]**: Can run in parallel (different files, no dependencies)
+- **[Story]**: Which user story this task belongs to
+
+---
+
+## Phase 1: User Story 1 — Previous/Next Page Navigation (Priority: P1) 🎯 MVP
+
+**Goal**: Replace the "Load more" append pattern with discrete Previous/Next page navigation, a "Page N of M" indicator, and a total image count. Page size changes from 50 to 24.
+
+**Independent Test**: With at least 25 images in the library, open `/`. Confirm 24 images are shown, a "Page 1 of N" indicator is visible, "Previous" is absent, and "Next" is present. Click "Next" — confirm the grid is replaced (not appended) with the next 24 images and the indicator updates. Click "Previous" — confirm the first page returns. Apply a tag filter — confirm the page resets to 1.
+
+- [X] T001 [US1] Write tests in `ui/src/app/library/library.component.spec.ts` covering: (1) page indicator text "Page 1 of N" renders when totalPages > 1; (2) total count text renders (e.g. "143 images"); (3) "Next" button present when not on last page; (4) "Previous" button absent on first page; (5) "Previous" present and "Next" absent on last page; (6) no pagination controls rendered when all images fit on one page (total ≤ 24); (7) clicking "Next" calls `imageService.list` with offset=24; (8) clicking "Previous" from page 2 calls `imageService.list` with offset=0; (9) applying a filter resets to page 1 (offset=0). Run `ng test` and confirm the new tests FAIL (implementation pending).
+
+- [X] T002 [US1] Update `ui/src/app/library/library.component.ts`: (a) change `private readonly limit = 50` to `private readonly limit = 24`; (b) remove `hasMore` property and `loadMore()` method; (c) add properties `currentPage = 1`, `totalPages = 1`, `total = 0`; (d) rename/replace `load()` to call `imageService.list(this.activeFilters, this.limit, (this.currentPage - 1) * this.limit)` and on success set `this.images = res.items` (replace, not append), `this.total = res.total`, `this.totalPages = Math.ceil(res.total / this.limit)`, clamp `currentPage` to `Math.max(1, Math.min(this.currentPage, this.totalPages))`; (e) add `nextPage()` that increments `currentPage` and calls `load()`; (f) add `prevPage()` that decrements `currentPage` and calls `load()`; (g) in `applyFilter()`, reset `this.currentPage = 1` before calling `load()`; (h) replace the `<button class="load-more">` element in the template with a pagination bar: a "Previous" button bound to `(click)="prevPage()"` disabled/hidden when `currentPage === 1`, a "Page {{ currentPage }} of {{ totalPages }}" span, a "Next" button bound to `(click)="nextPage()"` disabled/hidden when `currentPage === totalPages`, and place a total count element showing "{{ total }} images" **outside** the pagination bar and outside the `*ngIf="totalPages > 1"` guard so it always renders when images exist (FR-003, SC-002); wrap only the Previous button, page indicator span, and Next button inside `*ngIf="totalPages > 1"`. Run `ng test` and confirm T001 tests pass.
+
+**Checkpoint**: US1 complete. Library shows paginated results with Previous/Next controls and page indicator.
+
+---
+
+## Phase 2: User Story 2 — Page State in URL (Priority: P2)
+
+**Goal**: Persist the current page number in the URL query string (`?page=N`) so that the URL is bookmarkable and the browser Back button works.
+
+**Independent Test**: Navigate to page 3. Copy the URL (should contain `?page=3`). Open in a new tab — confirm page 3 loads directly. Press browser Back — confirm page 2 is shown. Navigate to `/?page=9999` — confirm page 1 loads without error.
+
+- [X] T003 [US2] Add tests to `ui/src/app/library/library.component.spec.ts` covering: (1) on init with `?page=2` in queryParamMap, `currentPage` is set to 2 and `list` is called with `offset=24`; (2) on init with `?page=9999` and total of 48 images, `currentPage` is clamped to page 1; (3) `nextPage()` calls `router.navigate` with `queryParams: { page: 2 }` and `queryParamsHandling: 'merge'`; (4) `applyFilter()` calls `router.navigate` with `queryParams: { page: 1 }` and `queryParamsHandling: 'merge'`. Run `ng test` and confirm new tests FAIL.
+
+- [X] T004 [US2] Update `ui/src/app/library/library.component.ts`: (a) in `ngOnInit`, after reading the `tags` param, read `const pageParam = this.route.snapshot.queryParamMap.get('page')` and set `this.currentPage = pageParam ? Math.max(1, parseInt(pageParam, 10)) : 1` (out-of-range clamping happens after load when totalPages is known); (b) update `nextPage()` and `prevPage()` to call `this.router.navigate([], { queryParams: { page: this.currentPage }, queryParamsHandling: 'merge' })` after updating `currentPage`; (c) update `applyFilter()` to call `this.router.navigate([], { queryParams: { page: 1, tags: tags.join(',') || null }, queryParamsHandling: 'merge' })` when resetting to page 1 (pass `null` for tags to remove param when empty); (d) after load resolves and `totalPages` is known, clamp `currentPage` to `Math.min(this.currentPage, Math.max(1, this.totalPages))` and if clamped, call navigate to correct the URL. Run `ng test` and confirm T003 tests pass.
+
+**Checkpoint**: US2 complete. Page state persists in URL; Back button and direct links work.
+
+---
+
+## Phase 3: Polish & Cross-Cutting Concerns
+
+- [X] T005 Run `ng lint` on `ui/src/app/library/library.component.ts` and fix any issues; confirm `ng test` passes with all existing and new tests green; manually verify all quickstart.md scenarios in a browser (pagination controls, URL state, tag filter reset, single-page no-controls, out-of-range URL, empty state).
+
+---
+
+## Dependencies & Execution Order
+
+- T001 before T002 (write failing tests before implementation)
+- T002 before T003 (US2 tests build on US1 implementation)
+- T003 before T004 (write failing tests before implementation)
+- T004 before T005 (polish after full implementation)
+
+### Execution Order Summary
+
+```
+Step 1: T001   (US1: failing tests)
+Step 2: T002   (US1: implementation — tests turn green)
+Step 3: T003   (US2: failing tests)
+Step 4: T004   (US2: implementation — tests turn green)
+Step 5: T005   (polish: lint + manual verification)
+```
+
+---
+
+## Implementation Strategy
+
+### MVP (US1 only)
+
+1. T001–T002 — page navigation controls, limit change, replace append
+2. **STOP and VALIDATE**: open browser, confirm pagination controls appear and work
+3. Deploy if ready
+
+### Full Delivery
+
+1. T001–T002 (US1) → validate
+2. T003–T004 (US2) → validate URL state
+3. T005 (polish) → ship