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/components/SegmentedButton.svelte

@@ -8,7 +8,7 @@
       class="rounded-lg font-medium
         {size === 'small' ? 'px-2.5 py-1 text-[12px]' : 'px-3.5 py-[6px] text-[12px]'}
         {value === option
-          ? 'bg-accent text-bg shadow-[0_1px_4px_rgba(214,132,80,0.3)]'
+          ? 'bg-accent text-bg shadow-[var(--shadow-accent-pill)]'
           : 'text-text-secondary hover:text-text hover:bg-hover'}"
       style="transition-duration: var(--duration-ui)"
       onclick={() => value = option}