fix(mail): sync Object error, MIME subject decoding, sticky compact pagination

- Fix sync error: backend returns {synced, error?} not {success, synced_count}; frontend now checks error field
- Decode MIME encoded-words (=?UTF-8?Q?...?=) in backend during IMAP sync for subject/from/to/cc headers
- Add frontend decodeMimeHeader() utility for already-stored encoded subjects (MailList + MailDetail)
- Make pagination sticky at bottom of mail list (flex-col h-full, ul scrolls, pagination flex-shrink-0)
- Make pagination compact: smaller padding (px-1.5 py-0.5), text-xs, smaller icons, tighter spacing

frontend/src/pages/Mail.tsx

@@ -599,11 +599,16 @@ export function MailPage() {
           if (!selectedAccountId) return;
           setIsSyncing(true);
           try {
-            await triggerSync(selectedAccountId);
-            await loadMails();
-            toast.success(t('mail.syncSuccess'));
-          } catch (err) {
-            toast.error(err instanceof Error ? err.message : String(err));
+            const result = await triggerSync(selectedAccountId);
+            if ((result as any)?.error) {
+              toast.error((result as any).error);
+            } else {
+              await loadMails();
+              toast.success(t('mail.syncSuccess'));
+            }
+          } catch (err: any) {
+            const msg = err?.message || err?.detail || (typeof err === 'string' ? err : 'Sync failed');
+            toast.error(msg);
           } finally {
             setIsSyncing(false);
           }