feat(web): add frontend composer history for loaded session messages

[techotaku39]

Summary

• Add a frontend-only composer history triggered by # or ＃ at the absolute beginning of the composer.
• Search currently loaded user messages from the active session locally, without adding a backend API, database change, or cross-session query.
• Add ArrowUp/ArrowDown history navigation while preserving native multiline cursor movement and existing @, /, and $ autocomplete behavior.
• Support both textarea and rich composer inputs across desktop and mobile layouts.
• Restore only message text for attachment-bearing entries without displaying attachment names or attempting to restore files.
• Allow Ctrl/Cmd+Enter to send a literal #... value when Enter is configured for newline mode.
• Dismiss history candidates when whitespace immediately follows the history prefix.

Validation

• bun typecheck — passed.
• bun run test:e2e -- terminal-wrap-fidelity.spec.ts — passed, 2/2.
• Related Web tests — passed, 24/24.
• Live Playwright smoke — passed for #/＃ activation, # dismissal, and text-only restoration of attachment-bearing entries.
• bun run build — passed.

Related Issues

Fixes #1652

AI Disclosure

Implemented with assistance from OpenAI Codex using GPT-5.6.