feat(theme): pick terminal and UI fonts from the installed families (#196)

[kipavy]

Follows #199. Together they close #196.

Why

The font pickers offered two bundled presets and a free-text box. A family typed into that box was accepted whether or not it existed on the machine, and an unresolvable family silently falls through to the engine's proportional default — cells measured from a serif face, letters painted in it. That is exactly what #196 looks like, and nothing in the UI ever said the font had not been found.

#199 makes that failure degrade to a monospace face instead of a proportional one. This PR stops it happening in the first place.

What

Backend. A new list_system_fonts command enumerates installed families with fontdb. It scans the OS font directories in pure Rust, so it needs no fontconfig, CoreText or DirectWrite linkage and stays out of the cross-compile images' way. Faces are deduplicated into one row per family, reported under the English name record because that is what CSS matches on, and flagged monospace if any face in the family claims fixed pitch.

Frontend. The picker keeps the bundled presets pinned at the top, then lists the installed families with a search box, each row previewed in its own face. The terminal picker filters to monospaced families, with a Show all fonts toggle — some fonts misreport fixed pitch, and Nerd Fonts' non-Mono variants legitimately report proportional. Picking a family stores 'Family', monospace (or sans-serif for the UI picker), so the generic tail from #199 is present from the start. The free-text box stays as an escape hatch and now warns when the family it holds is not installed.

Bundled webfonts (JetBrains Mono, Source Code Pro) ship inside the app rather than being installed, so fontdb cannot see them — they stay pinned as presets and are never reported missing.

Strings added to all four locales (en/fr/ru/zh); the locale diffs are additive only.

Verification

Driven live in the headless container with the Meslo Nerd Fonts installed:

step	result
Picker open	Presets on top, then installed families, each row previewed in its own font
Monospace filter	Only DejaVu Sans Mono and the Meslo Mono variants; DejaVu Sans/Serif and the non-Mono Meslo variants hidden
Show all fonts	Proportional families appear; toggle flips to "Show monospace fonts only"
Pick MesloLGS Nerd Font Mono	Stored as 'MesloLGS Nerd Font Mono', monospace
Custom MesloLGS NF	Warns: "MesloLGS NF" isn't installed — the terminal will fall back to another font

That last case is the likely shape of the original report: powerlevel10k's installer ships the family as MesloLGS NF, not the Nerd Fonts v3 name MesloLGS Nerd Font Mono.

• tsc --noEmit clean.
• Frontend suite green: 519 files, 3977 tests.
• Rust --lib green: 349 passed, 0 failed. (A fresh worktree fails 3 commands::plugins tests until pnpm build:plugins has run — unrelated to this change.)
• cargo fmt --check clean.

Notes for review

• The monospaced flag comes from post.isFixedPitch and PANOSE, which is why the escape hatch exists rather than a hard filter.
• Enumeration parses every font on disk, so the result is cached for the session and resolved off the UI thread; a failure yields an empty list and the picker still offers presets plus free text.