UI overhaul Phase 4: shared overlays (palette, review, context menu, delete, theme popover, help)

[Ed-Barnes937]

Stacked on #18 (Phase 3). Do not merge yet. Base is ui-overhaul-p3-center.

Reworks the shared overlays. Built via recon → 4 sequential single-writer sub-steps → parallel verify, then a test-maintenance pass.

What's here (overlay code)

• Command palette — Sessions + Commands sections, blurred scrim, ⌘/placeholder/esc header. Provider model + fuzzy score preserved.
• Help — two-column keyboard grid (no keybinding changes).
• Context menu — brief's order; Kill — stop process (yellow, keeps the session) and Delete session… (red, removes it) are now distinct actions.
• Delete-confirm dialog — red ⌦, mono session name, branch-deletion warning; confirming routes through the existing optimistic removal (vanishes from Console and Board, counts recount, active tab advances).
• Code review — staged comments render as mauve you/staged bubbles; Apply (n) → reflects the staged count and closes the overlay on success (returns to the workspace).
• Theme picker — top-right anchored popover listing all themes with an active ✓ + follow-system toggle and live preview.

Small brief-sanctioned tweak: the active check always marks the resolved theme (even in system mode), and a per-appearance command seeds a concrete highlight.

Test maintenance (second commit)

Updated page objects + iwft scenarios to the new DOM/behavior. Note: 3 of these (sidebar grouping control, liveness dots, comment/blocked chips) trace to Phase 2 (#17) — that PR's sidebar redesign changed the DOM and the iwft suite wasn't re-run there; they're fixed here so the stack tip is green. The rest are Phase 4 (theme popover, review label/close).

Verification

• npm run typecheck clean · npm test 49/49 · npm run test-iwft 51/51.
• Verify panel (functional/regression + idiom/token-purity vs working tree): pass; no raw hex/rgba consumers; no Rust changes.

🤖 Generated with Claude Code