collapse cross-account dupes, Lato UI font, mail-list selection & density tuning

Backend:
- Collapse identical cross-account deliveries in a thread: new messages.delivery_id
  (migration 0011) preserves the real Message-ID even when message_id_hdr is
  synthesized (§7.1); get_thread groups by it and reports also_in_account_ids; the
  list count counts distinct deliveries. Backfill script for existing rows.

Frontend:
- Bundle Lato (SIL OFL) as the UI font; unread mail-list rows use Black weight.
- Mail-list selection: hover-avatar checkbox, tri-state "select all", bulk
  Archive/Pin/Delete (Restore/Delete in Archive/Trash/Spam) with a count in the
  reading pane; unread blue dot; "also delivered to" note in the thread.
- Spacing/size tuning: sender-column width, icon gaps, avatar size, bar heights,
  and a round of -1px font sizes across the sidebar, mail list, and reading pane.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01U72FZiF1W1pb8XxyWBTREA

frontend/tailwind.config.js

@@ -4,6 +4,20 @@ export default {
   content: ["./index.html", "./src/**/*.{ts,tsx}"],
   theme: {
     extend: {
+      fontFamily: {
+        sans: ['"Lato"', "system-ui", "-apple-system", '"Segoe UI"', "Roboto", "sans-serif"],
+      },
+      // Every named text size bumped +2px over Tailwind's defaults (line-heights too).
+      fontSize: {
+        xs: ["14px", "18px"],
+        sm: ["16px", "22px"],
+        base: ["18px", "26px"],
+        lg: ["20px", "30px"],
+        xl: ["22px", "30px"],
+        "2xl": ["26px", "34px"],
+        "3xl": ["32px", "38px"],
+        "4xl": ["38px", "42px"],
+      },
       // Semantic tokens backed by CSS variables (see index.css). Using these
       // instead of raw grays lets every surface switch with the theme.
       // Make every `border`/`divide` utility theme-aware by default, so a bare