Phase 1.2: session login + empty HTMX dashboard

- Session login for the single user Sam: server-side session store with an
  HMAC-signed session cookie, POST /login, POST /logout, and double-submit
  CSRF on every form post. Unauthenticated requests to / and /partials/*
  redirect to /login?next=...
- Empty dashboard shell in the Family Home Lab design system: left nav
  (category and folder placeholders), centre list, right detail pane.
- Copy tokens.css and the self-hosted Inter fonts from the family-home-lab
  design system into web/static; vendor htmx 2.0.4. No cross-project imports
  at runtime.
- Dark mode via prefers-color-scheme plus a persisted toggle; phone-first
  layout with 44px touch targets, hairline borders, 8px grid, pill CTAs.
- Config: EMAIL_AGENT_PASSWORD for the login password, falling back to
  EMAIL_AGENT_SESSION_SECRET so one configured secret is enough.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

web/templates/inbox.html

@@ -0,0 +1,5 @@
+<div class="empty-state">
+  <div class="empty-title">No messages yet</div>
+  <p class="empty-copy">Mail fetch arrives in Phase 2. New messages will appear here, grouped by category, with a recommended action on each row.</p>
+  <span class="pill">IMAP scan not connected</span>
+</div>