feat: Gist — browser-local code snippet manager

[Copilot]

Adds a Gist tool for saving, organizing, and viewing code snippets entirely in localStorage.

Data layer

• src/types/gist.ts — Gist shape: { id, title, language, content, createdAt, updatedAt }
• src/hooks/use-gists.ts — CRUD + batch-delete over localStorage, state synced on every mutation

UI components (radix-ui primitives, matching project style)

• dialog.tsx, select.tsx, checkbox.tsx, label.tsx, badge.tsx

Gist page (/gist)

• Create / Edit — optional title, 23-language dropdown, monospace textarea; form opens in a Dialog
• Syntax highlighting — react-syntax-highlighter (Prism, oneDark) with line numbers, rendered on row expand
• Copy — per-snippet clipboard button with async error handling
• Search — filters by title, content, or language in real time
• Batch delete — per-row checkboxes + select-all across filtered results; confirmation dialog before destructive action

Routing

• New /gist route in App.tsx; "Storage > Gist" entry added to sidebar