fix(jellyfin): inline-validate the off-media config folder field (like Plex)

[stevezau]

The off-media "Jellyfin config folder" settings field had no inline validation — only the Setup Health tab checked it (#274). The Plex field validates live; this adds the Jellyfin equivalent so the field shows valid/invalid as you type (exactly what was expected when you flagged it).

What changed

• New endpoint POST /api/settings/validate-jellyfin-config-folder (mirrors validate-plex-config-folder): returns {exists, valid_jellyfin_structure, writable, detail, error}. Not restricted to a fixed root (Jellyfin config is an arbitrary admin mount); admin-only, read-only stat, null-byte guarded.
• Shared logic: extracted looks_like_jellyfin_config_dir() so the field validator and the Setup Health probe use identical checks (can never disagree).
• Frontend: the field is wired to the live validator (input + browse-pick + on-open when off-media is on) → shows "Valid Jellyfin config folder" / a clear error. Added the feedback spans + has-validation to the input-group so messages render.

Tests

valid-via-data/, valid-via-plugins/, not-found, wrong-folder (media dir) → error, empty → neutral, valid-but-read-only → flagged. Architecture Review run — no HIGH; the MED (read-only branch coverage) fixed before commit.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 97.05882% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 84.00%. Comparing base (8ae5d02) to head (03709ce).
⚠️ Report is 1 commits behind head on dev.

Files with missing lines	Patch %	Lines
media_preview_generator/web/routes/api_settings.py	95.45%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #275   +/-   ##
=======================================
  Coverage   83.99%   84.00%           
=======================================
  Files          89       89           
  Lines       19640    19670   +30     
=======================================
+ Hits        16496    16523   +27     
- Misses       3144     3147    +3     

Files with missing lines	Coverage Δ
...dia_preview_generator/output/jellyfin_trickplay.py	90.75% <100.00%> (+0.44%)	⬆️
media_preview_generator/servers/jellyfin.py	83.26% <100.00%> (ø)
media_preview_generator/web/routes/api_settings.py	79.83% <95.45%> (+0.60%)	⬆️

... and 2 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.