feat(frontend): redesign settings/common with Aurora cassettes

Splits the monolithic settings page into 6 focused glass components matching
the polish of the recently redesigned settings/backup page.

- SettingsHero: PageHeader with 4 live status pills (URL host, timezone +
  ticking clock, locale codes, log severity tinted by level)
- IdentityCassette: groups External URL + Timezone + Locales as numbered
  rows; URL field gains copy + open chips and a mint border when valid
- TelegramCassette: webhook secret with show/hide toggle and verified
  status chip; cache TTL/max as oversized mono numerals with humanized
  previews ("720 hrs -> 30d")
- CacheLedger: mirrors BackupLedger -- big total, gradient capacity meter,
  tone-edged URL/Asset bucket rows colored by oldest entry age
- LoggingCassette: per-module overrides become tone-edged chips with
  severity-colored level borders; raw-text fallback behind toggle; live
  ACTIVE preview line
- SaveBar: sticky dirty-aware footer with citrus pulse, italic message,
  and Discard/Save (only renders when settings differ from baseline)

No backend changes -- same /settings and /settings/telegram-cache/* endpoints.

frontend/src/routes/settings/SettingsHero.svelte

@@ -0,0 +1,94 @@
+<script lang="ts">
+	import { onMount, onDestroy } from 'svelte';
+	import { t } from '$lib/i18n';
+	import PageHeader, { type HeaderPill } from '$lib/components/PageHeader.svelte';
+
+	type Tone = 'mint' | 'sky' | 'orchid' | 'coral' | 'citrus' | 'primary';
+
+	interface Settings {
+		external_url: string;
+		timezone: string;
+		supported_locales: string;
+		log_level: string;
+		log_format: string;
+	}
+
+	interface Props {
+		settings: Settings;
+	}
+
+	let { settings }: Props = $props();
+
+	// Live tick so the timezone pill shows the current local HH:MM.
+	let now = $state(new Date());
+	let tick: ReturnType<typeof setInterval> | null = null;
+	onMount(() => { tick = setInterval(() => { now = new Date(); }, 30_000); });
+	onDestroy(() => { if (tick) clearInterval(tick); });
+
+	function fmtClock(tz: string): string {
+		try {
+			return new Intl.DateTimeFormat('en-GB', {
+				timeZone: tz || 'UTC',
+				hour: '2-digit',
+				minute: '2-digit',
+				hour12: false,
+			}).format(now);
+		} catch { return '--:--'; }
+	}
+
+	function hostFromUrl(url: string): string {
+		if (!url) return '';
+		try { return new URL(url).host; }
+		catch { return url.replace(/^https?:\/\//, '').replace(/\/$/, ''); }
+	}
+
+	function localeCount(csv: string): number {
+		if (!csv) return 0;
+		return csv.split(',').map(s => s.trim()).filter(Boolean).length;
+	}
+
+	const SEVERITY_TONE: Record<string, Tone> = {
+		DEBUG: 'sky',
+		INFO: 'mint',
+		WARNING: 'citrus',
+		ERROR: 'coral',
+	};
+
+	const pills = $derived.by<HeaderPill[]>(() => {
+		const out: HeaderPill[] = [];
+
+		const host = hostFromUrl(settings.external_url);
+		out.push(host
+			? { label: host, tone: 'sky' }
+			: { label: t('settings.heroNoUrl') }
+		);
+
+		const tz = settings.timezone || 'UTC';
+		out.push({ label: `${tz} · ${fmtClock(tz)}`, tone: 'primary' });
+
+		const locales = settings.supported_locales || '';
+		const count = localeCount(locales);
+		out.push({
+			label: count > 0
+				? locales.split(',').map(s => s.trim()).filter(Boolean).map(s => s.toUpperCase()).join(' · ')
+				: t('settings.heroNoLocales'),
+			tone: 'orchid',
+		});
+
+		const lvl = (settings.log_level || 'INFO').toUpperCase();
+		out.push({
+			label: `${lvl} · ${settings.log_format || 'text'}`,
+			tone: SEVERITY_TONE[lvl] ?? 'mint',
+		});
+
+		return out;
+	});
+</script>
+
+<PageHeader
+	title={t('settings.title')}
+	emphasis={t('settings.titleEmphasis')}
+	description={t('settings.description')}
+	crumb={t('crumbs.systemConfiguration')}
+	{pills}
+/>