Show the theme selector as icons instead of text

The toolbar's theme-cycling button ("Theme: Auto") and the Settings
panel's Auto/Hell/Dunkel segmented control now use icons -- a sun, a
moon, and a half-filled circle for "follows system" -- drawn in this
project's existing hand-drawn SVG icon set (ToolbarIcon.vue) rather than
a new icon system or emoji. Tooltips/aria-labels keep the full text for
accessibility; the toolbar button's click-to-cycle behavior is
unchanged.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/components/SettingsPanel.vue

@@ -11,6 +11,7 @@ import { useTheme } from '../composables/useTheme';
 import { NUMBER_LOCALES, NUM_FORMATS } from '../store/format';
 import type { CellFormat, HAlign } from '../store/types';
 import { DEFAULT_GRID_SIZE, MIN_GRID_SIZE, MAX_GRID_SIZE } from '../store/types';
+import ToolbarIcon from './ToolbarIcon.vue';
 
 const store = useDocument();
 const { mode: themeMode } = useTheme();
@@ -512,9 +513,30 @@ function resetDoc() {
         <label class="f">
           <span>Erscheinungsbild</span>
           <div class="seg">
-            <button :class="{ on: themeMode === 'system' }" @click="themeMode = 'system'">Auto</button>
-            <button :class="{ on: themeMode === 'light' }" @click="themeMode = 'light'">Hell</button>
-            <button :class="{ on: themeMode === 'dark' }" @click="themeMode = 'dark'">Dunkel</button>
+            <button
+              :class="{ on: themeMode === 'system' }"
+              title="Auto (folgt dem System)"
+              aria-label="Auto (folgt dem System)"
+              @click="themeMode = 'system'"
+            >
+              <ToolbarIcon name="auto" />
+            </button>
+            <button
+              :class="{ on: themeMode === 'light' }"
+              title="Hell"
+              aria-label="Hell"
+              @click="themeMode = 'light'"
+            >
+              <ToolbarIcon name="sun" />
+            </button>
+            <button
+              :class="{ on: themeMode === 'dark' }"
+              title="Dunkel"
+              aria-label="Dunkel"
+              @click="themeMode = 'dark'"
+            >
+              <ToolbarIcon name="moon" />
+            </button>
           </div>
         </label>
         <label class="f">