Fix tab close modal, section headers, and credential storage

Tab close confirmation:
- Add is-visible to both backdrop AND modal element — the modal CSS
  starts with visibility:hidden/opacity:0, so the modal was invisible
  despite the backdrop blur appearing

Section headers (sidebar + profile editor):
- Add mousedown preventDefault() on all clickable section headers —
  WKWebView initiates a text selection gesture on mousedown for divs,
  swallowing the first click and requiring a double-click
- Fix sidebar delete profile to use custom modal instead of blocked
  window.confirm()

Credential storage:
- buildSecretCallback now respects profile's secretMode field:
  - 'saved': checks keychain/vault first, prompts if missing, then
    persists to keychain (preferred) or encrypted vault
  - 'session-only': caches in memory per-session, prompts once
  - 'ask-every-time': always prompts
- Previously the callback always prompted and never saved
- Change default secretMode from 'ask-every-time' to 'saved' in both
  Go defaults and JS frontend defaults

frontend/src/js/tabs.js

@@ -172,10 +172,14 @@ class TabManager {
             `;
             backdrop.appendChild(modal);
             document.body.appendChild(backdrop);
-            requestAnimationFrame(() => backdrop.classList.add('is-visible'));
+            requestAnimationFrame(() => {
+                backdrop.classList.add('is-visible');
+                modal.classList.add('is-visible');
+            });
 
             const close = (val) => {
                 backdrop.classList.remove('is-visible');
+                modal.classList.remove('is-visible');
                 setTimeout(() => backdrop.remove(), 200);
                 resolve(val);
             };