fix(web): scale Files tree with interface font size

[Umais-Adeed]

What changed

The Files tree was overriding its font size with a fixed 12px, so it ignored Settings → Appearance → Interface font size. This changes the override to 0.75rem, preserving the existing 12px default at the 16px interface setting while allowing the tree to scale with the interface.

Why

Fixes the Files-tree typography bug reported in #7957 without adding another setting or changing the compact default.

Verification

• vp fmt --check apps/web/src/components/files/FileBrowserPanel.tsx
• vp lint --report-unused-disable-directives apps/web/src/components/files/FileBrowserPanel.tsx
• vp run --filter @t3tools/web typecheck
• Manual local web verification with a disposable T3 home: the tree measured 12px at Interface 16px and 15px at Interface 20px.
• The change is limited to one CSS custom-property value in FileBrowserPanel.tsx.

UI evidence

Before/after screenshots were captured at the same 20px interface setting: the old tree remains 12px, while the patched tree scales to 15px.

Before

After

Fixes #7957

Generated with GPT-5.6-Luna in T3 Code using the Codex harness.

[macroscopeapp]

Approvability

Verdict: Skipped

Macroscope did not run approvability analysis for this PR. Macroscope could not determine whether this PR modifies its approvability configuration, so the PR was not approved automatically. A PR that may change the rules that govern approval is never approved automatically.

[shivamhwp]

Note: GPT-6 on behalf of shivam (@shivamhwp).

This fix needs to follow the stylesheet extraction in #9330. FileBrowserPanel.tsx now imports PIERRE_TREE_UNSAFE_CSS from apps/web/src/pierre-tree-theme.ts, where the override is still 12px; the same stylesheet is used by DiffFileTree. The current PR conflicts with main in the removed local CSS block.

Move the 0.75rem change into that shared stylesheet when updating the branch, and keep both callers on it. This preserves the 12px default and lets the Files tree and diff/PR tree reach 15px at Interface 20.