feat(workspaces): rework tile-grid visuals — Start-screen polish

The home canvas now reads as a real Windows 8 Start screen instead of a
cramped, edge-hugging list:

- Geometry: the wide unit is DERIVED from the canvas height (4 rows fill
  it: unit=height/5 clamped to 112-184px, gap=unit/9, margin=unit/3 clamped
  24-64px) — tiles are no longer tiny, and the tile field always carries a
  proportional outer margin (never compressed against the left edge). Tall
  mode keeps fluid columns with a taller 148px row + 12px gap.
- Tiles take a per-workspace SATURATED tone (new pure tileTone(id): FNV-1a
  hash → one of the six theme-semantic colors) with tone-content text —
  the Win8 colored-tile identity. Starred is now a ★ badge (a tile keeps
  its own color), not a gray-tint override.
- The tile face is the Win8 layout: small uppercase slug caption at the top,
  a bold span-aware title (lg → 3xl across 1×1 → 4×2), and a single-line
  stat row pinned to the bottom (count compacts to 'conv.' on 1×1 tiles,
  truncation-safe so narrow tiles never clip).
- Selection is a high-contrast offset ring (visible on any tile color);
  tiles lift on hover; the pan arrow is a solid elevated circle.
- Header gains a 'WORKSPACES' kicker; empty/loading states are centered
  and styled for the Start-screen canvas.

Persisted data (order + spans only) and all interaction semantics
(drag/FLIP, long-press, action bar, resize picker, create popover) are
untouched. Tests: geometry derivation/clamping, tileTone determinism,
wide-vs-compact count label — 1664 green.

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