feat(email): drafts, search, bulk read/unread, configurable backfill

Gmail client enhancements in apps/x, bundling four independent features:

- Drafts: save/autosave, update, delete, and list Gmail drafts. The
  composer autosaves to a real Gmail draft (debounced ~1.5s) while
  typing, reuses the thread's existing draft so edits update in place,
  flushes a final save on close, and deletes the draft on discard. Adds
  isDraft/draftId to the shared thread types. New core helpers:
  saveThreadDraft, deleteThreadDraft, listDraftThreads,
  buildDraftSnapshot, buildRawMimeMessage.

- Search: searchThreads(query, {limit}) backed by an on-disk snapshot
  cache (read/writeSearchSnapshot). Extracts parseThreadSnapshot as the
  shared parse core reused by both the cache-building sync and search.

- Read state: markThreadRead now takes a `read` flag so it toggles
  read/unread; new markSectionRead marks a whole section
  (important/other) read/unread and returns the affected count.

- Backfill: the onboarding/recovery sync is now bounded by a
  configurable thread COUNT instead of a fixed 7-day window. New
  gmail_sync_config.ts (getMaxEmails/setMaxEmails) backed by
  ~/.rowboat/config/gmail_sync.json, default 500, clamped to 1-5000;
  seeded on first run.

New IPC channels: gmail:saveDraft, gmail:deleteDraft, gmail:getDrafts,
gmail:search, gmail:markSectionRead (plus a `read` field on
gmail:markThreadRead).

apps/x/packages/shared/src/blocks.ts

@@ -124,6 +124,8 @@ export const GmailThreadMessageSchema = z.object({
   bodyHeight: z.number().int().positive().optional(),
   attachments: z.array(GmailAttachmentSchema).optional(),
   messageIdHeader: z.string().optional(),
+  // Set on the unsent draft message within a thread (used by the Drafts view).
+  isDraft: z.boolean().optional(),
 });
 
 export type GmailThreadMessage = z.infer<typeof GmailThreadMessageSchema>;
@@ -134,6 +136,9 @@ export const GmailThreadSchema = EmailBlockSchema.extend({
   unread: z.boolean().optional(),
   importance: z.enum(['important', 'other']).optional(),
   gmail_draft: z.string().optional(),
+  // Gmail-side draft id, present on entries returned by the Drafts list so the
+  // composer can update/delete that exact draft.
+  draftId: z.string().optional(),
   messages: z.array(GmailThreadMessageSchema),
 });
 

apps/x/packages/shared/src/ipc.ts

@@ -205,6 +205,56 @@ const ipcSchemas = {
       error: z.string().optional(),
     }),
   },
+  'gmail:saveDraft': {
+    req: z.object({
+      // Existing Gmail draft to update; omitted on first save (creates a new one).
+      draftId: z.string().min(1).optional(),
+      threadId: z.string().min(1).optional(),
+      // Recipients may be blank for a draft (unlike a send).
+      to: z.string().optional(),
+      cc: z.string().optional(),
+      bcc: z.string().optional(),
+      subject: z.string(),
+      bodyHtml: z.string(),
+      bodyText: z.string(),
+      inReplyTo: z.string().optional(),
+      references: z.string().optional(),
+      attachments: z
+        .array(
+          z.object({
+            filename: z.string(),
+            mimeType: z.string(),
+            contentBase64: z.string(),
+          }),
+        )
+        .optional(),
+    }),
+    res: z.object({
+      draftId: z.string().optional(),
+      error: z.string().optional(),
+    }),
+  },
+  'gmail:deleteDraft': {
+    req: z.object({ draftId: z.string().min(1) }),
+    res: z.object({ ok: z.boolean(), error: z.string().optional() }),
+  },
+  'gmail:getDrafts': {
+    req: z.object({}),
+    res: z.object({
+      threads: z.array(GmailThreadSchema),
+      error: z.string().optional(),
+    }),
+  },
+  'gmail:search': {
+    req: z.object({
+      query: z.string(),
+      limit: z.number().int().positive().optional(),
+    }),
+    res: z.object({
+      threads: z.array(GmailThreadSchema),
+      error: z.string().optional(),
+    }),
+  },
   'gmail:getConnectionStatus': {
     req: z.object({}),
     res: z.object({
@@ -235,9 +285,13 @@ const ipcSchemas = {
     res: z.object({ ok: z.boolean(), error: z.string().optional() }),
   },
   'gmail:markThreadRead': {
-    req: z.object({ threadId: z.string().min(1) }),
+    req: z.object({ threadId: z.string().min(1), read: z.boolean().optional() }),
     res: z.object({ ok: z.boolean(), error: z.string().optional() }),
   },
+  'gmail:markSectionRead': {
+    req: z.object({ section: z.enum(['important', 'other']), read: z.boolean() }),
+    res: z.object({ ok: z.boolean(), count: z.number().int().nonnegative(), error: z.string().optional() }),
+  },
   'gmail:saveMessageHeight': {
     req: z.object({
       threadId: z.string().min(1),