R8a: composer must never overlap the minimap or the token counter

[dovvnloading]

Problem

The composer island's width formula only reserved a thin margin from the minimap, and relied on a bare 7px vertical gap from the token counter - a razor-thin technical non-overlap, not a real design margin. At common window widths the composer's higher z-index let it visibly paint over both corners' chrome.

Change

Three independent, structural guarantees instead of one tuned distance:

1. .app-composer-layer now reserves real horizontal space on both sides (left: 230px for the token counter's measured footprint, right: 232px for the minimap's) and centers the card within that narrowed box via flex - not the full viewport - so centering can never land it in territory either neighbor already owns.
2. bottom: 64px still gives the token-counter side a second, independent vertical guarantee on top of that.
3. Both the token counter and the minimap now sit on a new --gl-z-canvas-hud tier (21) that outranks the composer's app-layer tier (20) regardless of position - so even if the geometry above were ever wrong, neither can be painted over.

Test plan

• Frontend: tsc --noEmit clean, lint 0 errors, 794 tests passed, build succeeds
• Backend: 995 passed (unaffected, frontend-only change)
• Verified live with real measured pixel gaps (not just "no overlap"): 26px clearance from the token counter and 19px from the minimap at the exact width the collision was first reported at, and confirmed no overlap at the app's real minimum window width (960px, per graphlink_desktop.py's own min_size)