refactor(theme): tokenise hard-coded amber shadows and reconcile duplicate @font-face declarations

Hard-coded rgba(214,132,80,X) shadows were tied to the dark-theme accent
and stayed off-hue in light mode (where the accent is a darker #a3683a).
Six tokens added to app.css @theme and mirrored in the light-theme block
plus design-system/colors_and_type.css:

  --shadow-accent-sm     0 0 8px   0.25 alpha  Toggle on-state
  --shadow-accent-md     0 4px 16px 0.3       ModelDownloader card
  --shadow-accent-lg     0 4px 20px 0.3       DictationPage record button
  --shadow-accent-glow   0 0 8px   0.4        progress-bar glow
  --shadow-accent-pill   0 1px 4px 0.3        SegmentedButton pill
  --shadow-accent-raised 0 2px 8px 0.2        FilesPage browse tile

Migrated seven sites: Toggle, SegmentedButton, ModelDownloader (x2),
FilesPage (x2), DictationPage record button, SettingsPage locale pill.
Focus-ring shadows (0_0_0_3px_rgba(...,0.1)) left alone — handled in
parallel via the --accent-shadow-focus token.

@font-face: app.css and colors_and_type.css both declare the same five
fonts. Duplication is unavoidable because preview/*.html loads the
design-system file directly without Vite, so it cannot share the
runtime declarations. font-weight ranges and font-style reconciled to
match exactly (Atkinson 200-800 not 400-700, JetBrains gains font-style:
normal); src URLs intentionally differ (root-absolute vs relative).
Comments added to both copies explaining the contract.

src/lib/pages/DictationPage.svelte

@@ -834,7 +834,7 @@
               ? 'bg-warning opacity-60 cursor-wait'
               : !tauriRuntimeAvailable
                 ? 'bg-bg-elevated text-text-tertiary cursor-not-allowed'
-                : 'bg-accent hover:bg-accent-hover shadow-[0_4px_20px_rgba(214,132,80,0.3)]'}"
+                : 'bg-accent hover:bg-accent-hover shadow-[var(--shadow-accent-lg)]'}"
         onclick={toggleRecording}
         disabled={modelLoading || !tauriRuntimeAvailable}
         aria-label={page.recording ? "Stop recording" : "Start recording"}