feat(web): add file filter autocomplete to chat input

[mitre88]

Summary

• add file filter autocomplete suggestions directly in web chat input
• detect active file: token while typing and show a suggestions popover
• fetch synced files from /api/content/computer and rank suggestions by starts-with then contains
• insert selected suggestion as file:"" into the current chat message

Why

Issue #1182 asks for file: autocomplete in chat so users can scope context to synced files without typing full filepaths manually.

Verification

• bun run lint (web app)
• bunx tsc --noEmit (web app)

Closes #1182