fix: UI polish — overflow, placeholders, dashboard provider card

- Fix bot card header overflow by replacing "Sync with Telegram" text
  button with icon button, add flex-wrap
- Rename sync button label to "Sync Commands"
- Remove decorative dashes from selector placeholders (— X — → X)
- Show selected provider name/icon in dashboard stat card when global
  provider filter is active
- Add selector placeholder convention to frontend-architecture.md

.claude/docs/frontend-architecture.md

@@ -42,3 +42,9 @@ Shared entities use a `$state`-based cache layer in `frontend/src/lib/stores/`:
 2. Add `export const fooCache = createEntityCache<Foo>('/foo');` to `caches.svelte.ts`
 3. Add `fooCache.clear()` to `clearAllCaches()`
 4. In page components: replace `let foo = $state<Foo[]>([])` with `let foo = $derived(fooCache.items)` and replace `api('/foo')` with `fooCache.fetch()`
+
+## UI Conventions
+
+### Selector Placeholders
+
+**IMPORTANT**: Selector/dropdown placeholder text must be plain and simple — no decorative dashes or special characters. Use `Select provider...` or `Tracking Configs`, NOT `— Select provider —` or `-- Tracking Configs --`. The i18n keys already follow this convention; never wrap them with `'— ' + t('key') + ' —'` in Svelte templates.