fix(web): drop the taskbar theme label, align window padding to p-2

- Taskbar: the theme toggle is icon-only now, matching the Settings
  button beside it. The theme name moves into the title/aria-label so
  an icon-only control still has an accessible name and the current
  theme stays discoverable on hover.
- Pages: Fleet, Knowledge, Learning, Ops and Signals used p-4 (or
  p-4 md:p-6) while Tasks used p-2, so windows didn't line up. All now
  p-2. App Store and Settings are deliberately untouched — they have no
  root padding, using a full-bleed header whose border spans the window;
  insetting them would break that.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/src/lib/components/desktop-shell/Taskbar.svelte

@@ -109,12 +109,12 @@
   <div class="flex shrink-0 items-center gap-0.5">
     <button
       type="button"
-      class="flex items-center justify-center gap-1.5 rounded-md p-1.5 text-muted-foreground hover:bg-muted hover:text-foreground"
+      class="flex items-center justify-center rounded-md p-1.5 text-muted-foreground hover:bg-muted hover:text-foreground"
       onclick={() => toggleTheme()}
-      title="Cycle theme"
+      title="Cycle theme ({THEME_LABELS[getTheme()]})"
+      aria-label="Cycle theme, currently {THEME_LABELS[getTheme()]}"
     >
       <PaletteIcon class="size-4" />
-      <span class="hidden text-xs sm:inline">{THEME_LABELS[getTheme()]}</span>
     </button>
     <button
       type="button"