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/profiles.js

@@ -16,8 +16,8 @@ const DEFAULTS = {
         protocol: 'ssh',
         ssh: {
             authMethods: ['publickey', 'password', 'keyboard-interactive'],
-            secretMode: 'ask-every-time',
-            keySecretMode: 'ask-every-time',
+            secretMode: 'saved',
+            keySecretMode: 'saved',
             hostKeyPolicy: 'ask',
             terminalType: 'xterm-256color',
             keepaliveInterval: 30,
@@ -459,6 +459,9 @@ class ProfileEditor {
             }
         }
 
+        // Prevent mousedown from starting a text selection gesture in WKWebView,
+        // which would swallow the first click and require a double-click.
+        header.addEventListener('mousedown', (e) => e.preventDefault());
         header.addEventListener('click', () => {
             collapsed = !collapsed;
             content.style.display = collapsed ? 'none' : 'block';

frontend/src/js/sidebar.js

@@ -117,6 +117,9 @@ class Sidebar {
             <span>${escapeHtml(title)}</span>
             <span class="section-count">${profiles.length}</span>
         `;
+        // Prevent mousedown from starting a text selection gesture in WKWebView,
+        // which would swallow the first click and require a double-click.
+        header.addEventListener('mousedown', (e) => e.preventDefault());
         header.addEventListener('click', () => this.toggleSection(key));
         section.appendChild(header);
 
@@ -305,7 +308,9 @@ class Sidebar {
      * @private
      */
     async _deleteProfile(profile) {
-        const ok = confirm(`Delete "${profile.displayName || profile.host}"?\nThis cannot be undone.`);
+        // Use a custom confirmation modal — window.confirm() is silently
+        // blocked in WKWebView and always returns false.
+        const ok = await this._confirmDelete(profile);
         if (!ok) return;
 
         try {
@@ -318,6 +323,52 @@ class Sidebar {
         }
     }
 
+    /**
+     * Shows a confirmation modal for profile deletion.
+     * @param {Object} profile
+     * @returns {Promise<boolean>}
+     * @private
+     */
+    _confirmDelete(profile) {
+        return new Promise((resolve) => {
+            const backdrop = createElement('div', 'modal-backdrop');
+            const modal = createElement('div', 'modal modal-sm');
+            const label = escapeHtml(profile.displayName || profile.host || 'Untitled');
+            modal.innerHTML = `
+                <div class="modal-header">
+                    <h3 class="modal-title">Delete Profile</h3>
+                    <button class="btn btn-icon btn-xs modal-close">&times;</button>
+                </div>
+                <div class="modal-body">
+                    <p style="font-size:var(--text-sm);color:var(--text-secondary)">
+                        Delete <strong>${label}</strong>? This cannot be undone.
+                    </p>
+                </div>
+                <div class="modal-footer">
+                    <button class="btn btn-sm confirm-cancel">Cancel</button>
+                    <button class="btn btn-sm btn-danger confirm-ok">Delete</button>
+                </div>
+            `;
+            backdrop.appendChild(modal);
+            document.body.appendChild(backdrop);
+            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);
+            };
+            modal.querySelector('.modal-close').addEventListener('click', () => close(false));
+            modal.querySelector('.confirm-cancel').addEventListener('click', () => close(false));
+            modal.querySelector('.confirm-ok').addEventListener('click', () => close(true));
+            backdrop.addEventListener('click', (e) => { if (e.target === backdrop) close(false); });
+        });
+    }
+
     // ── Filtering ────────────────────────────────────────────────────
 
     /**

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);
             };