fix(categories): correct all seven smart-folder bugs at the source (PHASE 2)

Root causes + fixes:
- Archive leaked Sent mail: filter was just 'not in inbox'. Added ExcludeGmailLabels;
  Archive now excludes SENT/DRAFT/SPAM/TRASH/CHAT by label.
- Read Later / Pinned / Unlabelled returned EVERYTHING (no folder case -> default all).
  Pinned = IsImportant; Read Later = new local IsReadLater marker (+toggle endpoints);
  Unlabelled = HasUserLabels=false.
- Trash & Spam always empty: IncludeSpamTrash=false + sync never set IsTrashed nor created
  EmailLabel rows. Now fetches spam/trash, sets IsTrashed from the TRASH label, and links
  every message to its Gmail labels (also fixes Sent/Spam/category-by-label/Unlabelled).
- Old Mail: now strictly older than 6 months (was 1 year).

Guardrail: sync upsert is now idempotent (drops prior copy before reinsert) so re-sync
can't duplicate a message or leave stale labels. 4 new filter tests; 65 backend + frontend
green; migration is a single non-destructive column add.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/api/client.js

@@ -94,13 +94,22 @@ function folderToRequest(slug, page, pageSize) {
     case 'starred':   return { ...base, isStarred: true };
     case 'sent':      return { ...base, gmailLabel: 'SENT' };
     case 'drafts':    return { ...base, gmailLabel: 'DRAFT' };
-    case 'archive':   return { ...base, isInInbox: false, isTrashed: false };
+    // Archive = filed away: not in inbox, not trashed, and NOT Sent/Spam/Draft/Chat.
+    // Excluding those labels stops sent mail leaking into Archive.
+    case 'archive':   return { ...base, isInInbox: false, isTrashed: false, excludeGmailLabels: ['SENT', 'DRAFT', 'SPAM', 'TRASH', 'CHAT'] };
     case 'spam':      return { ...base, gmailLabel: 'SPAM' };
     case 'trash':     return { ...base, isTrashed: true };
+    // Pinned = Gmail's "Important" marker (a real per-message flag), not "everything".
+    case 'pinned':    return { ...base, isImportant: true };
+    // Read Later = only emails the user explicitly flagged (local marker).
+    case 'readlater': return { ...base, isReadLater: true };
+    // Unlabelled = emails not filed under any user-created label.
+    case 'unlabelled': return { ...base, hasUserLabels: false };
     // ── Special filters ──
     case 'large':     return { ...base, minSizeBytes: 5_000_000 };
+    // Old Mail = strictly older than 6 months.
     case 'old': {
-      const d = new Date(); d.setFullYear(d.getFullYear() - 1);
+      const d = new Date(); d.setMonth(d.getMonth() - 6);
       return { ...base, to: d.toISOString().slice(0, 10) };
     }
     // ── Smart folders ──