feat(ui): add a logger to change log level and permit better diagnostics

in this case logs are added in the delta upload worker

you can increase verbosity from console via:

```javascript
// Usage:
oxi.setLogLevel('oxi:upload', 'debug')    // deep dive
oxi.setLogLevel('oxi:upload', 'warn')     // quiet
oxi.log.setLevel('debug')                  // everything to debug
```

values are stored in localstorage

example:

```
oxi.setLogLevel('oxi:upload', 'debug');
'oxi:upload → debug'
deltaUpload.ts:120 [f32ce1] delta start {file: 'Revue de presseg.odp', size: 23658927}
deltaUpload.ts:185 [f32ce1] worker: worker start {file: 'Revue de presseg.odp', size: 23658927}
deltaUpload.ts:186 [f32ce1] worker: wasm loaded
deltaUpload.ts:186 [f32ce1] worker: hashed — blake3=150edbc35f3475a0bdeecd15fe5278dda305ce3b27e53e9c40515a69f0b883f3 (76 chunks)
deltaUpload.ts:186 [f32ce1] worker: negotiate: 76 hashes → 76 missing, 0 dedup'd
deltaUpload.ts:186 [f32ce1] worker: chunk PUT: 28 chunks, 8825338 bytes
deltaUpload.ts:186 [f32ce1] worker: chunk PUT: 29 chunks, 8950230 bytes
deltaUpload.ts:186 [f32ce1] worker: chunk PUT: 19 chunks, 5883663 bytes
deltaUpload.ts:186 [f32ce1] worker: ✅ committed — uploaded 23 658 927 B (no dedup, blake3=150edbc35f3475a0bdeecd15fe5278dda305ce3b27e53e9c40515a69f0b883f3)
deltaUpload.ts:185 [f32ce1] worker: commit HTTP 201 {blake3: '150edbc35f3475a0bdeecd15fe5278dda305ce3b27e53e9c40515a69f0b883f3', uploadedBytes: 23658927, reusedBytes: 0, totalBytes: 23658927, attempt: 0}
deltaUpload.ts:213 [f32ce1] delta done {file: 'Revue de presseg.odp', blake3: '150edbc35f3475a0bdeecd15fe5278dda305ce3b27e53e9c40515a69f0b883f3', savedBytes: 0, uploadedBytes: 23658927}
```

frontend/src/hooks.client.ts

@@ -3,12 +3,58 @@
  *  - wires the API client's session-expired behaviour (clear store + redirect),
  *  - loads translations for the resolved locale.
  */
+import log from 'loglevel';
 import { setSessionExpiredHandler } from '$lib/api/client';
 import { initI18n } from '$lib/i18n/index.svelte';
 import { session } from '$lib/stores/session.svelte';
 import { seedNonceFromCookie } from '$lib/auth/dpop-proof';
 
+// DevTools shortcut: expose a small `oxi.*` helper on window so users
+// can toggle log levels from the browser console without needing to
+// import anything. Namespaces used today: `oxi:upload` (delta + direct
+// upload pipeline). Levels: 'trace' | 'debug' | 'info' | 'warn' | 'error' | 'silent'.
+// Choices persist to `localStorage['loglevel:<namespace>']` via loglevel.
+//
+// Usage:
+//   oxi.setLogLevel('oxi:upload', 'debug')    // deep dive
+//   oxi.setLogLevel('oxi:upload', 'warn')     // quiet
+//   oxi.log.setLevel('debug')                  // everything to debug
+declare global {
+	interface Window {
+		oxi?: {
+			log: typeof log;
+			setLogLevel: (namespace: string, level: log.LogLevelDesc) => string;
+			listLogLevels: () => Record<string, string>;
+		};
+	}
+}
+
 export async function init(): Promise<void> {
+	if (typeof window !== 'undefined') {
+		window.oxi = {
+			log,
+			// Return a confirmation string so the DevTools echo is a
+			// useful "worked → new level" signal instead of `undefined`.
+			setLogLevel(namespace, level) {
+				log.getLogger(namespace).setLevel(level);
+				return `${namespace} → ${level}`;
+			},
+			// Enumerate the levels loglevel has persisted so users can see
+			// what's currently set without opening the Application tab.
+			listLogLevels() {
+				const out: Record<string, string> = {};
+				if (typeof localStorage === 'undefined') return out;
+				for (let i = 0; i < localStorage.length; i++) {
+					const key = localStorage.key(i);
+					if (key?.startsWith('loglevel:')) {
+						out[key.slice('loglevel:'.length)] = localStorage.getItem(key) ?? '';
+					}
+				}
+				return out;
+			}
+		};
+	}
+
 	setSessionExpiredHandler(() => {
 		session.reset();
 		if (typeof window !== 'undefined') {