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),
 });