[codex] Add modular overview workspace layout

[tobomobo]

Summary

• add a reusable page workspace layout layer with snapped movement, resize constraints, collision push-down, z-order focus, add/remove, and reset helpers
• persist page workspace layouts in the existing local UI store keyed by book identity plus page id
• convert the Overview dashboard sections into configurable page-level widgets and seed the palette with planned report/source-funds/wallet placeholders
• keep the normal dashboard visually calm: grid, handles, remove controls, and widget palette only appear in Edit layout mode
• make drag/resize use a lightweight snapped preview outline and commit the real layout only on release
• document the V1 scope, UX guardrails, and follow-up proof criteria in the desktop plan/reference and TODO.md

Scope notes

This PR intentionally applies the workspace system to Overview first. Reports and Transactions can reuse the shared PageWorkspace layer in follow-up slices without reworking the layout engine.

The long-term UX bar is workflow composition, not movable cards for their own sake. Future expansion should wait until one concrete workflow proves the value, such as a transaction cleanup workstation with table/detail/filter/review-queue modules, or a tax review workstation with blockers, journal freshness, E 1kv preview, and export status.

Long-term UX guardrails

• ship opinionated presets for real jobs: tax review, transaction cleanup, reporting/export readiness, source-of-funds review, and wallet monitoring
• keep each page's widget catalog small and page-specific, adding modules only when they expose real state or actions
• keep normal mode static and quiet: no grid, handles, palette, or motion outside Edit layout
• preserve reset/default safety now, and add restore preset or save-as-preset later if users customize heavily
• enforce strong constraints: no overlapping modules, no unusably small widgets, no empty decorative dashboards
• add precise non-pointer controls before broad rollout: keyboard nudging, size presets, and tile commands such as half-width/full-width/move-to-top
• keep edit mode, persistence, reset behavior, and accessibility rules consistent across pages
• do not expand the pattern if the only gain is rearranging dashboard cards

Validation

• pnpm --dir ui-tauri exec vitest run src/lib/workspaceLayout.test.ts src/store/ui.test.ts
• pnpm --dir ui-tauri exec tsc -b --noEmit
• targeted eslint on changed UI/store/layout files
• pnpm --dir ui-tauri run build
• headless browser render check for normal and Edit layout modes on /overview
• ./scripts/quality-gate.sh