feat: show autocomplete suggestions for file query filters in web chat

[gnanam1990]

Summary

Fixes #1182

Adds file filter autocomplete to the Khoj web chat input. When a user types \ile:\ in the chat box, a dropdown appears showing their synced files. The list filters as they type, and clicking a suggestion completes the filter in the input.

How it works

1. User types \ile:\ in chat input
2. Fetches synced files from /api/content/files\
3. Shows filtered dropdown below the input
4. User selects a file → input updated with full path
5. Press Escape or click outside to dismiss

Changes

• Added \FileFilterAutocomplete.tsx\ component
• Updated chat input to detect \ile:\ trigger and show autocomplete

Testing

• Type \ile:\ in the chat input box
• Verify dropdown appears with synced files
• Type partial filename — verify list filters
• Click a suggestion — verify input is updated
• Press Escape — verify dropdown closes
• Keyboard navigation works (↑↓ arrows + Enter)

Related: The Obsidian plugin has this feature (PR #1128).
This brings the same UX to the web app.

cc @debanjum