[auth] Add "Lock this device" control to re-lock a kept-unlocked session (#51)

The opt-in "keep unlocked" cache had no user-facing way to undo it mid-session
(it cleared only on logout / clear-site-data). Add a "Lock this device" button to
the profile modal footer (desktop + mobile), shown only when the active account
has a cached key to forget (app.isSessionKeptUnlocked -> client.hasCachedUnlockedKey).

Clicking it runs app.lockKeptUnlockedSession(pubkey), which re-locks in one action
everything the signer unlocks:
  - forgets the cached key (both tiers) so future reloads re-prompt for the PIN,
  - drops the in-memory signer (logoutSigner) so the CURRENT session re-locks now
    (the next signing action prompts again via ensureEncryptionCapableSigner),
  - storageService.lock(pubkey), and
  - nwcSettingsService.clearCache() (re-hydrates from the signer next time).

Visibility refreshes on renderSavedProfiles + selectPane. Tests: 2 in
tests/profile-modal-controller.test.mjs (shown only when kept-unlocked; click locks
the active pubkey). Lint + build green.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

components/profile-modal.html

@@ -373,10 +373,31 @@
                 </svg>
                 Admin (Moderation)
               </button>
+              <button
+                id="profileMobileLockNowBtn"
+                type="button"
+                class="btn-ghost focus-ring w-full justify-start gap-2 px-3 py-2 text-xs font-medium mt-6 lg:hidden hidden"
+                title="Forget your unlocked key on this device and re-lock storage + wallet now"
+              >
+                <svg
+                  class="h-4 w-4"
+                  fill="none"
+                  viewBox="0 0 24 24"
+                  stroke="currentColor"
+                >
+                  <path
+                    stroke-linecap="round"
+                    stroke-linejoin="round"
+                    stroke-width="2"
+                    d="M12 15v2m-6 4h12a2 2 0 002-2v-6a2 2 0 00-2-2H6a2 2 0 00-2 2v6a2 2 0 002 2zm10-10V7a4 4 0 00-8 0v4h8z"
+                  />
+                </svg>
+                Lock this device
+              </button>
               <button
                 id="profileMobileLogoutBtn"
                 type="button"
-                class="btn-ghost focus-ring w-full justify-start gap-2 px-3 py-2 text-xs font-medium mt-6 lg:hidden"
+                class="btn-ghost focus-ring w-full justify-start gap-2 px-3 py-2 text-xs font-medium mt-2 lg:hidden"
                 data-variant="critical"
               >
                 <svg
@@ -1943,6 +1964,14 @@
 
       <!-- Modal Footer -->
       <div class="hidden lg:flex items-center justify-end gap-3 border-t border-border/60 p-4">
+        <button
+          id="profileLockNowBtn"
+          type="button"
+          class="btn-ghost focus-ring hidden"
+          title="Forget your unlocked key on this device and re-lock storage + wallet now"
+        >
+          Lock this device
+        </button>
         <button id="profileLogoutBtn" class="btn focus-ring" data-variant="danger">
           Logout
         </button>