feat(desktop): crash viewer panel with global panic capture

[gandarfh]

Adds a local crash viewer so a user can inspect panics after a restart.

Backend (httui-core, merged separately)

Pins httui-core to the merged crash_log file store (3a36cc7).

Desktop

• Global panic hook installed at startup: captures location, payload, and
  backtrace, writes a crash file via httui_core::crash_log, then chains
  to the default hook. Best-effort — never re-panics.
• 3 Tauri commands in commands/settings.rs (list / read / clear)
• New "Crashes" settings tab: source badge + timestamp + summary per row,
  click to view the full body, "Clear all" control, empty state

Crash logs live in ~/.config/httui/crashes/ as plain text — local only,
nothing uploaded. The source field is generic so the language server can
write into the same directory (follow-up).

Tests

• crash wrappers, CrashesSection render (list / select / clear / empty),
  SettingsDrawer tab switching
• Full suite green; every touched file >= 80% line coverage