fix(editors): hoist ToolbarButton (static-components ×9) + restore live toolbar active states

[tbcsec]

Batch 1 of 3 for #38.

Lint fix: ToolbarButton was declared inside both editors' render bodies — a new component type every render, so React remounted the button subtree each parent render (react-hooks/static-components, 9 warnings → 0). The two definitions were byte-identical; they're now one shared module-scope component. Warning count: 28 → 18, other rules untouched.

Bug found during live verification: toolbar active-state highlights have been frozen since the TipTap 3 migration (#69) — v3's useEditor defaults shouldRerenderOnTransaction to false (marked legacy-to-be-removed), so editor.isActive(...) evaluated in render never refreshed. Fixed the v3-idiomatic way: useEditorState with a selector over the active flags, re-rendering exactly when a flag flips (React-Compiler-friendly, per-keystroke-re-render-free).

Verification (demo stack, both editors):

• CollabNote (challenge scratchpad): B/I/Code/List render from the shared component, room binds Live, highlight tracked bold→unbold→rebold across selections, focus stayed in the editor after toolbar clicks.
• RichTextEditor (challenge editor): clean 9/9 pass — plain selection inactive, bold + H2 apply and highlight, all toggles restored, document left as found. No console errors.
• tsc clean · eslint 0 errors / 18 warnings (the remaining Burn down the 27 react-hooks v7 (React Compiler) lint warnings #38 batches) · vitest 130/130 · production build untested locally (CI covers it).

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