feat(portal-shell): align theme switcher trigger with locale switcher shape

The theme switcher used to render as a round 44×44 icon button while
the locale switcher next to it (in the footer's device-prefs cluster
since #162) renders as a small chip: icon + label + chevron. Two
controls sitting side-by-side with two different visual shapes read
as two unrelated widgets instead of one family of preferences.

This PR aligns the theme trigger on the locale chip shape:

  Before:  [☀]                       (round icon button, 44×44)
  After:   [☀ System ▾]              (chip — icon + label + chevron)

The leading icon stays driven by `currentIcon()` so it still flips
between sun / moon / monitor as the user selects light / dark /
system — that visual feedback was the one reason the original icon-
only design existed and it's preserved.

Implementation:

* `theme-switcher.html` swaps the Tailwind utility-class icon button
  for a `.theme-switcher__trigger` chip with three children — the
  current-mode icon (size 14), the localised mode label, and a
  `chevron-down` (size 12). Same children layout as
  `.locale-switcher__trigger`.

* `theme-switcher.scss` adds `.theme-switcher__trigger` mirroring
  `.locale-switcher__trigger` rule-for-rule: same chip metrics
  (min-height 2.75rem for the AAA 44×44 tap target via vertical-
  padding overflow inside the thin footer), same hover/focus tokens,
  same dark-mode swap. Two copies rather than a shared `_chip-
  trigger.scss` partial — two switchers in one app is below the
  "three similar things" threshold from CLAUDE.md where extraction
  starts paying off. Promotes when a third switcher lands.

No spec changes: the existing assertions check
`button[aria-haspopup="menu"]` + the aria-label content, both of
which are preserved. `triggerAriaLabel()` is unchanged.

Verification:
  * 43 portal-shell specs pass.
  * `pnpm nx run portal-shell:build:production` (i18n-strict) green
    — confirms the `theme.mode.{light,dark,system}` labels are
    already translated (they were used in the menu items before;
    the trigger just reuses the same `currentLabel()` computed).

apps/portal-shell/src/app/components/theme-switcher/theme-switcher.html

@@ -1,10 +1,12 @@
 <button
   type="button"
-  class="inline-flex h-11 w-11 items-center justify-center rounded-full text-gray-600 transition-colors hover:bg-gray-100 hover:text-brand-primary-500 focus-visible:outline-2 focus-visible:outline-offset-2 focus-visible:outline-brand-primary-500 dark:text-gray-300 dark:hover:bg-gray-800 dark:hover:text-brand-primary-300"
+  class="theme-switcher__trigger"
   [cdkMenuTriggerFor]="themeMenu"
   [attr.aria-label]="triggerAriaLabel()"
 >
-  <lib-icon [name]="currentIcon()" />
+  <lib-icon [name]="currentIcon()" [size]="14" />
+  <span>{{ currentLabel() }}</span>
+  <lib-icon name="chevron-down" [size]="12" />
 </button>
 
 <ng-template #themeMenu>

apps/portal-shell/src/app/components/theme-switcher/theme-switcher.scss

@@ -3,6 +3,47 @@
 // when the labels grow under translation. dark-mode surface mirrors
 // the rest of the shell (gray-900 background, gray-800 border).
 
+// Trigger mirrors `.locale-switcher__trigger` — same chip shape,
+// same hover / focus tokens, same AAA-friendly 44×44 tap target
+// via vertical-padding overflow inside the thin footer. The two
+// switchers sit side-by-side in the footer's device-prefs cluster
+// (#162), so a single visual family across the pair reads as one
+// component. The leading icon stays driven by `currentIcon()` so a
+// sun / moon / monitor glyph still signals which mode is selected.
+.theme-switcher__trigger {
+  display: inline-flex;
+  align-items: center;
+  gap: 0.375rem;
+  min-height: 2.75rem;
+  margin: -0.5rem 0;
+  padding: 0 0.5rem;
+  border-radius: 0.25rem;
+  background: transparent;
+  border: 0;
+  color: inherit;
+  font-size: inherit;
+  font-weight: 500;
+  cursor: pointer;
+  transition:
+    background-color 0.12s ease-out,
+    color 0.12s ease-out;
+
+  &:hover {
+    background-color: #f3f4f6;
+    color: var(--color-brand-primary-500);
+
+    :where(.dark) & {
+      background-color: #1f2937;
+      color: var(--color-brand-primary-300);
+    }
+  }
+
+  &:focus-visible {
+    outline: 2px solid var(--color-brand-primary-500);
+    outline-offset: 2px;
+  }
+}
+
 .theme-switcher__menu {
   min-width: 11rem;
   margin-top: 0.25rem;