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, 88 insertions, 0 deletions

diff --git a/src/features/workspaces/ui/ResizePicker.test.ts b/src/features/workspaces/ui/ResizePicker.test.ts
new file mode 100644
index 0000000..796a556
--- /dev/null
+++ b/src/features/workspaces/ui/ResizePicker.test.ts
@@ -0,0 +1,88 @@
+import { render, screen } from "@testing-library/svelte";
+import userEvent from "@testing-library/user-event";
+import { describe, expect, it, vi } from "vitest";
+import { geometryFor } from "../logic/tile-layout";
+import ResizePicker from "./ResizePicker.svelte";
+
+const WIDE = geometryFor(1920, 1080);
+const TALL = geometryFor(390, 844);
+
+function props() {
+  return {
+    onPreview: vi.fn(),
+    onCommit: vi.fn(),
+    onCancel: vi.fn(),
+  };
+}
+
+describe("ResizePicker", () => {
+  it("renders one small square per cap (4×2 wide, 2×2 tall)", () => {
+    const p = props();
+    const wide = render(ResizePicker, {
+      props: { geometry: WIDE, initial: { colSpan: 1, rowSpan: 1 }, ...p },
+    });
+    expect(screen.getAllByRole("button")).toHaveLength(8);
+    wide.unmount();
+
+    render(ResizePicker, {
+      props: { geometry: TALL, initial: { colSpan: 1, rowSpan: 1 }, ...p },
+    });
+    expect(screen.getAllByRole("button")).toHaveLength(4);
+  });
+
+  it("hovering square (r, c) previews that rectangle", async () => {
+    const user = userEvent.setup();
+    const p = props();
+    render(ResizePicker, {
+      props: { geometry: WIDE, initial: { colSpan: 1, rowSpan: 1 }, ...p },
+    });
+
+    await user.hover(screen.getByRole("button", { name: "Resize to 3 by 2" }));
+    expect(p.onPreview).toHaveBeenCalledWith({ colSpan: 3, rowSpan: 2 });
+  });
+
+  it("clicking a square commits that size", async () => {
+    const user = userEvent.setup();
+    const p = props();
+    render(ResizePicker, {
+      props: { geometry: WIDE, initial: { colSpan: 1, rowSpan: 1 }, ...p },
+    });
+
+    await user.click(screen.getByRole("button", { name: "Resize to 2 by 1" }));
+    expect(p.onCommit).toHaveBeenCalledWith(2, 1);
+  });
+
+  it("arrow keys move within the picker, Enter commits, Escape cancels", async () => {
+    const user = userEvent.setup();
+    const p = props();
+    render(ResizePicker, {
+      props: { geometry: WIDE, initial: { colSpan: 2, rowSpan: 1 }, ...p },
+    });
+
+    await user.keyboard("{ArrowRight}");
+    expect(p.onPreview).toHaveBeenLastCalledWith({ colSpan: 3, rowSpan: 1 });
+    await user.keyboard("{ArrowDown}");
+    expect(p.onPreview).toHaveBeenLastCalledWith({ colSpan: 3, rowSpan: 2 });
+    await user.keyboard("{ArrowLeft}");
+    expect(p.onPreview).toHaveBeenLastCalledWith({ colSpan: 2, rowSpan: 2 });
+
+    await user.keyboard("{Enter}");
+    expect(p.onCommit).toHaveBeenCalledWith(2, 2);
+    expect(p.onCancel).not.toHaveBeenCalled();
+
+    await user.keyboard("{Escape}");
+    expect(p.onCancel).toHaveBeenCalledTimes(1);
+  });
+
+  it("leaving the picker clears the preview", async () => {
+    const user = userEvent.setup();
+    const p = props();
+    render(ResizePicker, {
+      props: { geometry: WIDE, initial: { colSpan: 1, rowSpan: 1 }, ...p },
+    });
+
+    await user.hover(screen.getByRole("button", { name: "Resize to 4 by 2" }));
+    await user.unhover(screen.getByRole("button", { name: "Resize to 4 by 2" }));
+    expect(p.onPreview).toHaveBeenLastCalledWith(null);
+  });
+});