feat(web): migrate dashboard to WidgetGrid with layout persistence

Replace the hardcoded dashboard layout with the WidgetGrid system.
The dashboard now loads/saves user layouts via the UserLayout API
and creates a default layout on first visit with all 7 widgets.

Changes:
- Add layout API client (fetchDefaultLayout, createLayout, updateLayout)
- Add default layout constant with 7-widget arrangement (12-col grid)
- Update BaseWidget + WidgetGrid to use CSS variables for theme compat
- Add responsive container width measurement via ResizeObserver
- Rewrite dashboard page to use WidgetGrid with edit mode toggle
- Update all tests for the new dashboard architecture

Ref: #488

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

docs/TASKS.md

@@ -8,8 +8,8 @@
 | TW-THM-001  | done        | Theme architecture — Create theme definition interface, theme registry, and 5 built-in themes (Dark, Light, Nord, Dracula, Solarized) as TS files          | #487  | web  | feat/ms18-theme-architecture     | TW-PLAN-001                                            | TW-THM-002,TW-THM-003                       | worker       | 2026-02-23 | 2026-02-23   | 30K      | ~15K | PR #493 merged                             |
 | TW-THM-002  | done        | ThemeProvider upgrade — Load themes dynamically from registry, apply CSS variables, support instant theme switching without page reload                    | #487  | web  | feat/ms18-theme-provider-upgrade | TW-THM-001                                             | TW-THM-003,TW-VER-002                       | worker       | 2026-02-23 | 2026-02-23   | 25K      | ~12K | PR #494 merged                             |
 | TW-THM-003  | done        | Theme selection UI — Settings page section with theme browser, live preview swatches, persist selection to UserPreference.theme via API                    | #487  | web  | feat/ms18-theme-selection-ui     | TW-THM-001,TW-THM-002                                  | TW-VER-002                                  | worker       | 2026-02-23 | 2026-02-23   | 25K      | ~10K | PR #495 merged                             |
-| TW-WDG-001  | not-started | Widget definition seeding — Seed 7 existing widgets into widget_definitions table with correct sizing constraints and configSchema                         | #488  | api  | TBD                              | TW-PLAN-001                                            | TW-WDG-002                                  | worker       | —          | —            | 15K      | —    |                                            |
-| TW-WDG-002  | not-started | Dashboard → WidgetGrid migration — Replace hardcoded dashboard layout with WidgetGrid, load/save layout via UserLayout API, default layout on first visit  | #488  | web  | TBD                              | TW-WDG-001                                             | TW-WDG-003,TW-WDG-004,TW-WDG-005            | worker       | —          | —            | 40K      | —    |                                            |
+| TW-WDG-001  | done        | Widget definition seeding — Seed 7 existing widgets into widget_definitions table with correct sizing constraints and configSchema                         | #488  | api  | feat/ms18-widget-seed            | TW-PLAN-001                                            | TW-WDG-002                                  | worker       | 2026-02-23 | 2026-02-23   | 15K      | ~8K  | PR #496 merged                             |
+| TW-WDG-002  | in-progress | Dashboard → WidgetGrid migration — Replace hardcoded dashboard layout with WidgetGrid, load/save layout via UserLayout API, default layout on first visit  | #488  | web  | feat/ms18-widget-grid-migration  | TW-WDG-001                                             | TW-WDG-003,TW-WDG-004,TW-WDG-005            | worker       | 2026-02-23 | —            | 40K      | —    |                                            |
 | TW-WDG-003  | not-started | Widget picker UI — Drawer/dialog to browse available widgets from registry, preview size/description, add to dashboard                                     | #488  | web  | TBD                              | TW-WDG-002                                             | TW-VER-001                                  | worker       | —          | —            | 25K      | —    |                                            |
 | TW-WDG-004  | not-started | Widget configuration UI — Per-widget settings dialog using configSchema, configure data source/filters/colors/title                                        | #488  | web  | TBD                              | TW-WDG-002                                             | TW-VER-001                                  | worker       | —          | —            | 30K      | —    |                                            |
 | TW-WDG-005  | not-started | Layout management UI — Save/rename/switch/delete layouts, reset to default. UI controls in dashboard header area                                           | #488  | web  | TBD                              | TW-WDG-002                                             | TW-VER-001                                  | worker       | —          | —            | 20K      | —    |                                            |
@@ -26,9 +26,9 @@
 | Metric        | Value                     |
 | ------------- | ------------------------- |
 | Total tasks   | 16                        |
-| Completed     | 4 (PLAN-001, THM-001–003) |
-| In Progress   | 0                         |
-| Remaining     | 12                        |
-| PRs merged    | #493, #494, #495          |
+| Completed     | 5 (PLAN-001, THM-001–003, WDG-001) |
+| In Progress   | 0                                   |
+| Remaining     | 11                                  |
+| PRs merged    | #493, #494, #495, #496              |
 | Issues closed | —                         |
 | Milestone     | MS18-ThemeWidgets         |