fix(theme): WCAG-aware btn-primary-text token across all themes

Frontend (62 components, ~64 rules):
- Replace hardcoded `color: white` / `#fff` with `var(--btn-primary-text)`
  in every rule that paints text on `var(--accent-primary)`. Covers in-rule
  cases (BulkActionBar, GenreFilterButton, KeybindingsSettings, settings
  views, login, modal primary buttons, wizard steps, etc.) plus descendant
  selectors (`.parent.selected .child`).
- Semi-transparent rgba(255,255,255,X) over selected accent rows now use
  `color-mix(in srgb, var(--btn-primary-text) X%, transparent)` so the mute
  level inherits the per-theme token color.

Theme tokens in app.css (--btn-primary-text fixes):
- rose-pine-dawn: #faf4ed → #575279 (was the bg color, ratio 2.60 FAIL;
  now 5.71 AA against coral accent)
- kurosaki:       #fbf9f2 → #262424 (was the bg color, ratio 1.76 FAIL;
  now 8.33 AA against gold accent)
- snow-storm:     #eceff4 → #2e3440 (was the bg color, ratio 3.50 AA-Large;
  now 6.13 AA against blue accent)
- stratego:       #0a0a0a → #ffffff (visual choice; 4.14 AA-Large is
  acceptable for button labels and matches palette intent)

Auto-theme generator (src-tauri/src/auto_theme/generator.rs):
- pick_btn_text_for_accent_set(): WCAG-aware FG selection that holds
  across accent / hover / active states. Replaces the broken
  `luminance < 0.5` threshold which produced 2.0:1 ratios on mid-luminance
  accents (Catppuccin lavender, Dracula purple, rose-pine coral).
- ensure_text_contrast_target(text, bg, is_dark, target): generalize the
  iterative contrast helper to accept a target ratio. Apply to
  text_secondary (4.5) and text_muted (3.0).
- Add --btn-danger-text and --btn-warning-text tokens via
  pick_btn_text_for_bg for future status-button consumers.

Misc fixes:
- SettingsView.svelte: extract initialTitlebarMode to const, avoids the
  Svelte 5 state_referenced_locally warning.
- QueuePanel.svelte: wrap .stop-after-marker in :global() (the class is
  forwarded to a Svelte component child, not a native element) and fix
  var(--accent) typo to the correct var(--accent-primary).

Tests: 13 generator tests pass (8 prior + 5 new). svelte-check 0 errors,
31 warnings (down from 33).

cc

src/routes/+page.svelte

@@ -7303,7 +7303,7 @@
   .create-modal .primary-btn {
     padding: 10px 20px;
     background: var(--accent-primary);
-    color: #ffffff;
+    color: var(--btn-primary-text);
     border: none;
     border-radius: 8px;
     font-size: 14px;