feat(palette): command palette overlay bound to cmd/ctrl+k on home

[srfwb]

Summary

Turns the inert search bar on Home into a real command palette. Cmd+K / Ctrl+K (⌘K on macOS, Ctrl K elsewhere) or a click on the Home search bar opens a backdropped overlay with grouped results. Lives behind a partial scaffold that was sitting untracked in src/palette/ — this PR plugs the sources, filter, renderer, shortcut listener and styles around it.

Scope for v1: Home only. The IDE toolbar placeholder is untouched — a follow-up PR can decide whether to bridge it or drop it.

Groups shown:

• Reprendre — top 5 projects by lastOpenedAt, minus the active one
• Fichiers — files from the active project's VFS (empty when no project is active)
• Commandes — typed registry (v1: "Nouveau projet")
• Leçons — single placeholder row that toasts "Bientôt disponible" on select

What changed

User-facing

• On Home, Cmd+K / Ctrl+K opens the palette. The badge next to the Home search bar adapts to the host OS (⌘K on Mac, Ctrl K elsewhere).
• Clicking the Home search bar also opens the palette — no more dead input.
• Typing filters live with case-insensitive substring match; the matched substring is highlighted with a <mark> amber tint.
• ↑ / ↓ navigate, Enter runs the selected row, Escape closes (focus returns to where it was via useModalA11y).
• Choosing a file switches the view to IDE and opens that file in one step.
• Choosing a project opens it (same path as clicking a project card).
• On IDE view the shortcut does nothing — the existing IDE shortcuts (Ctrl+N, Ctrl+W, Ctrl+Tab) stay untouched.

Internal

• src/palette/ — new module. types.ts grows PALETTE_GROUP_ORDER and PALETTE_GROUP_LABELS; paletteStore.ts (open/query singleton), formatShortcut.ts (OS-adaptive modifier), commands.ts (typed registry), sources.ts (aggregator reading live stores), filterPaletteItems.ts (pure substring matcher with title/subtitle ranking), highlightMatch.tsx (wraps the matched slice in <mark>), iconForFile.ts (path → glyph + tone), CommandPalette.tsx (overlay with grouped results, keyboard nav, useModalA11y), useCommandPaletteShortcut.ts (window keydown listener with view === "home" guard).
• src/App.tsx — renders <CommandPalette /> next to <Toasts /> and binds the shortcut hook.
• src/home/sections/HomeSearch.tsx — no more disabled input, now a clickable trigger that opens the palette. shortcutLabel("K") replaces the hardcoded Ctrl K.
• src/styles/global.css — new .palette-* class family (~300 lines). All token-based — new oklch values only for the per-type icon tones.
• Vitest coverage grows by 34 tests (130 total): helpers, registry, filter, highlight, format-shortcut, CommandPalette smoke.