fix: use theme color for file mention category icon

[wibus-wee]

Problem / pressure

The Files category in the mention menu used a file asset with a fixed blue-gray stroke, so it differed from the other category icons and ignored the menu's highlighted color.

Summary

Render the Files category with Lucide's monochrome File icon, using the same size, stroke width, and inherited menu color as the other categories. Individual file candidates retain their file-type icons.

Visual explanation

Simple change: one category glyph changes its rendering source; the menu structure and behavior are unchanged.

Before / after

Before	After
Files category stays blue-gray, including when highlighted.	Files category follows the theme and highlighted row color.
File candidates use file-type colors.	File candidates retain file-type colors.

Test plan

• Passed: git diff --check.
• Blocked: targeted mention-menu tests (vitest missing), pnpm check (tsgo missing), and pnpm format (oxfmt missing). This nested checkout has no installed dependencies.
• Ran pnpm run docs status and pnpm run docs check; the latter reports existing broken links, including absent ACP submodule files.
• Runtime visual verification was not performed.

[github-actions]

Desktop PR regression failed on commit 537b196e06c34f72ab6f06819d9da0396f68a67f.

• Workflow run: https://github.com/LodyAI/Lody/actions/runs/37129108162
• Failed scenarios indexed: 0
• Recording: 1/1

Recordings not attached

• Run evidence: failure-index.json is missing

The Actions artifact retains the complete trace, screenshots, logs, and runtime evidence.