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, 90 insertions, 0 deletions

diff --git a/src/features/workspaces/ui/HomeHeader.test.ts b/src/features/workspaces/ui/HomeHeader.test.ts
new file mode 100644
index 0000000..a0afc52
--- /dev/null
+++ b/src/features/workspaces/ui/HomeHeader.test.ts
@@ -0,0 +1,90 @@
+import { render, screen } from "@testing-library/svelte";
+import userEvent from "@testing-library/user-event";
+import { describe, expect, it, vi } from "vitest";
+import HomeHeader from "./HomeHeader.svelte";
+
+function props() {
+  return {
+    onCreate: vi.fn(async (_slug: string): Promise<{ ok: true }> => ({ ok: true })),
+    onOpenPlugins: vi.fn(),
+    onSort: vi.fn(),
+  };
+}
+
+describe("HomeHeader", () => {
+  it("renders the display title and the primary New workspace button", () => {
+    const p = props();
+    render(HomeHeader, { props: p });
+    expect(screen.getByRole("heading", { name: "Dispatch" })).toBeInTheDocument();
+    expect(screen.getByRole("button", { name: "New workspace" })).toBeInTheDocument();
+  });
+
+  it("the Plugins button renders only with the plugins port", () => {
+    const p = props();
+    const without = render(HomeHeader, { props: p });
+    expect(without.queryByRole("button", { name: "Plugins" })).toBeNull();
+    without.unmount();
+
+    render(HomeHeader, { props: { ...p, withPlugins: true } });
+    screen.getByRole("button", { name: "Plugins" }).click();
+    expect(p.onOpenPlugins).toHaveBeenCalledTimes(1);
+  });
+
+  it("the Sort overflow renders only when asked, and dispatches", async () => {
+    const user = userEvent.setup();
+    const p = props();
+    render(HomeHeader, { props: { ...p, withSort: true } });
+
+    await user.click(screen.getByRole("button", { name: "More actions" }));
+    await user.click(screen.getByRole("button", { name: "Sort by recent activity" }));
+    expect(p.onSort).toHaveBeenCalledTimes(1);
+  });
+
+  it("the Create button disables for an invalid slug (validation before submit)", async () => {
+    const user = userEvent.setup();
+    const p = props();
+    render(HomeHeader, { props: p });
+
+    await user.click(screen.getByRole("button", { name: "New workspace" }));
+    const input = screen.getByRole("textbox", { name: "New workspace" });
+    await user.type(input, "Invalid Slug!");
+    expect(screen.getByRole("button", { name: "Create" })).toBeDisabled();
+    expect(p.onCreate).not.toHaveBeenCalled();
+
+    // A valid slug enables it.
+    await user.clear(input);
+    await user.type(input, "my-workspace");
+    expect(screen.getByRole("button", { name: "Create" })).toBeEnabled();
+  });
+
+  it("submitting the form with Enter creates via the callback and closes the popover", async () => {
+    const user = userEvent.setup();
+    const p = props();
+    render(HomeHeader, { props: p });
+
+    await user.click(screen.getByRole("button", { name: "New workspace" }));
+    await user.type(screen.getByRole("textbox", { name: "New workspace" }), "my-workspace{Enter}");
+
+    expect(p.onCreate).toHaveBeenCalledWith("my-workspace");
+  });
+
+  it("a backend error keeps the form open with the message", async () => {
+    const user = userEvent.setup();
+    const onCreate = vi.fn(
+      async (): Promise<{ ok: false; error: string }> => ({ ok: false, error: "taken" }),
+    );
+    render(HomeHeader, { props: { ...props(), onCreate } });
+
+    await user.click(screen.getByRole("button", { name: "New workspace" }));
+    await user.type(screen.getByRole("textbox", { name: "New workspace" }), "my-workspace");
+    await user.click(screen.getByRole("button", { name: "Create" }));
+
+    expect(await screen.findByText("taken")).toBeInTheDocument();
+  });
+
+  it("compact mode collapses the primary button to an icon", () => {
+    const p = props();
+    render(HomeHeader, { props: { ...p, compact: true } });
+    expect(screen.getByRole("button", { name: "New workspace" })).toHaveTextContent("+");
+  });
+});