feat(workspaces): Win8 Start-style tile-grid home page

Rebuild the workspace home as a Windows 8 Start-screen-style tile grid,
replacing the max-w-3xl vertical card list.

Pure core (zero DOM/Svelte/fetch, fully unit-tested):
- logic/tile-layout.ts — geometryFor(width, height) resolves WIDE (4 fixed
  ~120px rows, column-wise flow, 4x2 span cap) or TALL (2 fixed columns,
  row-wise flow, 2x2 cap); pack() is the ONLY order->coordinates transform
  (greedy first-fit in fill order, hole-free, render-clamped per geometry);
  firstEmptySlot/insertAtFirstEmptySlot/mergeLayout/resizeTile/moveTile/
  sortByRecentActivity/sanitizeLayout. Persisted data is
  orientation-independent {version:1, tiles:[{id,colSpan,rowSpan,order}]} —
  order + spans only, never positions; the same data renders in either
  geometry (spans clamp for the render, persisted values untouched).
- logic/drag-controller.ts — pure pointer/press state machine: mouse
  threshold-drag + tap-open; touch long-press lifts + selects, pre-lift
  movement cancels so vertical scrolling never fights the gesture.

Shell:
- tile-layout-store.svelte.ts — thin reactive wrapper persisting through
  the injected createLocalStore adapter (dispatch.workspaces.tiles), ops:
  sync/resize/move/placeFirstEmpty/sortByRecentActivity.
- ui/HomeTileGrid.svelte — CSS-grid tile canvas (wide: horizontal scroll +
  snap + wheel pan + pan arrow + scroll indicator; tall: native vertical
  scroll), FLIP-animated reflow, drag ghost, transient resize preview.
- ui/HomeHeader.svelte — fixed chrome: Dispatch display title, New
  workspace popover (same slug validation; creates via create-on-miss and
  does NOT navigate — the new tile lands in the first empty slot in fill
  order and enters rename mode), global Plugins button, sort-by-recent
  overflow; compact icon buttons + bottom sheet on tall screens.
- ui/WorkspaceTile.svelte — tile face (title, /slug, conversation count,
  relative last-activity, live active dots, starred tint); inline rename
  with the old card's semantics.
- ui/TileActionBar.svelte — slides up on select (right-click / long-press):
  Open, Rename, Resize, Star/Unstar, Details, Plugins, Delete (old confirm
  + store call preserved).
- ui/ResizePicker.svelte — Word-style insert-table hover grid (wide 4x2 /
  tall 2x2) with arrow/Enter/Escape support and ghost preview on commit.
- ui/WorkspaceDetails.svelte — flyout holding the old card's default-cwd
  input + default-computer select (same store calls + error states).
- WorkspacesHome.svelte orchestrates; App.svelte untouched (same props).
- WorkspaceCard.svelte is removed (replaced by the tile system).

Manual tile order rules; starred workspaces get a tint (star is no longer
a sort key); workspaces unknown to the layout append at the end.

vitest: jsdom gains PointerEvent (MouseEvent-based) + Element.animate
stubs — the pointer logic is the outermost edge for the drag tests.

Verification: svelte-check 0/0, vitest 1655 passed (102 files, twice),
biome clean, vite build OK.

1 files changed, 29 insertions, 1 deletions

diff --git a/src/features/workspaces/index.ts b/src/features/workspaces/index.ts
index dab1dec..0aa7fca 100644
--- a/src/features/workspaces/index.ts
+++ b/src/features/workspaces/index.ts
@@ -1,5 +1,11 @@
 export type { WorkspaceHttp, WorkspaceResult } from "./adapter/http";
 export { createWorkspaceHttp } from "./adapter/http";
+export type {
+  DragEmit,
+  DragEvent as DragControllerEvent,
+  DragPhase,
+} from "./logic/drag-controller";
+export { nextDragPhase } from "./logic/drag-controller";
 export type { Route } from "./logic/route";
 export {
   DEFAULT_WORKSPACE_ID,
@@ -8,10 +14,32 @@ export {
   WORKSPACE_SLUG_RE,
   workspacePath,
 } from "./logic/route";
+export type {
+  GridGeometry,
+  Placement,
+  TileGeometryMode,
+  TileLayoutData,
+  TileLayoutEntry,
+} from "./logic/tile-layout";
+export {
+  clampSpan,
+  fillOrderKey,
+  firstEmptySlot,
+  geometryFor,
+  insertAtFirstEmptySlot,
+  mergeLayout,
+  moveTile,
+  pack,
+  resizeTile,
+  sanitizeLayout,
+  sortByRecentActivity,
+  sortTiles,
+} from "./logic/tile-layout";
 export { applyStarred, pageTitle, relativeTime, sortWorkspaces } from "./logic/view-model";
 export type { WorkspaceStore } from "./store.svelte";
 export { createWorkspaceStore } from "./store.svelte";
-export { default as WorkspaceCard } from "./ui/WorkspaceCard.svelte";
+export type { TileLayoutStore } from "./tile-layout-store.svelte";
+export { createTileLayoutStore, TILE_LAYOUT_STORAGE_KEY } from "./tile-layout-store.svelte";
 export { default as WorkspacesHome } from "./ui/WorkspacesHome.svelte";
 
 /** Public module manifest — aggregated by the shell's "Loaded Modules" view. */