Add self-service password change + forgot-password guidance

- Logged-in users can change their own password from a "Password" link
  in the top-right pill (dialog -> POST /api/auth/password, which requires
  the current password).
- Login page gains a "Forgot password?" link explaining that resets are
  admin-assisted (admins reset from the console). No SMTP, so no email
  reset flow yet.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

html/login.html

@@ -94,6 +94,13 @@
       <button id="submit" type="submit">Sign in</button>
     </form>
 
+    <p style="margin-top:1.25rem; text-align:center; font-size:0.8125rem;">
+      <a href="#" id="forgot-link" style="color:var(--cds-link-primary); text-decoration:none;">Forgot password?</a>
+    </p>
+    <div id="forgot-msg" style="display:none; margin-top:0.5rem; font-size:0.8125rem; color:var(--cds-text-secondary); background:var(--cds-layer-accent); border-left:3px solid var(--cds-link-primary); padding:0.75rem; border-radius:0 6px 6px 0;">
+      Password resets are handled by an administrator. Contact your project admin and they'll set a new one for you. Once you're signed in, you can change it yourself anytime from the menu in the top-right corner.
+    </div>
+
     <p class="foot">Authorized use only · BTG / Pilot</p>
   </main>