Phase 3: Session recording/playback and encrypted profile export/import

Session Recording & Playback:
- Add recording package with asciicast v2 format writer and reader
- Recorder captures terminal I/O with timestamps from SSH and Telnet sessions
- Input and output events recorded via hooks in session manager callbacks
- Recording store manages .cast files in ~/Library/Application Support/NexusTerminal/recordings
- Recording browser modal lists all saved recordings with metadata
- Full playback viewer with embedded xterm.js terminal, progress scrubber,
  play/pause/restart controls, and 0.5x-10x speed adjustment
- Recording indicator (pulsing red dot) shown on active tab during capture
- Keyboard shortcut Cmd+Shift+R to toggle recording on active session
- Tests for recording, store listing, deletion, and double-close safety

Encrypted Profile Export/Import:
- Add exportimport package with AES-256-GCM encryption (Argon2id key derivation)
- File format: version byte + 32-byte salt + 12-byte nonce + ciphertext
- Same Argon2id parameters as vault (128 MiB, 3 iterations, 4 threads)
- Export modal with profile selection checkboxes and password entry
- Import modal with password entry and conflict strategy (skip/overwrite/duplicate)
- Native save/open file dialogs for .nexus files
- Tests for round-trip encryption, wrong password rejection, corrupt files
- Replaced placeholder import/export with full encrypted implementation

UI:
- Added export and recordings buttons to sidebar footer
- Recording and playback CSS with charcoal design tokens
- Wired RecordingManager and ExportImportUI into app.js

frontend/index.html

@@ -17,6 +17,7 @@
     <link rel="stylesheet" href="/src/css/tabs.css">
     <link rel="stylesheet" href="/src/css/terminal.css">
     <link rel="stylesheet" href="/src/css/splitpane.css">
+    <link rel="stylesheet" href="/src/css/recordings.css">
     <link rel="stylesheet" href="/src/css/forms.css">
     <link rel="stylesheet" href="/src/css/modals.css">
     <link rel="stylesheet" href="/src/css/github.css">
@@ -61,7 +62,7 @@
                 <button class="btn btn-ghost btn-sm" id="btn-new-connection" title="New Connection">
                     <span>+ New</span>
                 </button>
-                <button class="btn btn-ghost btn-icon btn-sm" id="btn-import" title="Import">
+                <button class="btn btn-ghost btn-icon btn-sm" id="btn-import" title="Import Profiles">
                     <!-- Upload icon -->
                     <svg xmlns="http://www.w3.org/2000/svg" width="14" height="14" viewBox="0 0 24 24"
                          fill="none" stroke="currentColor" stroke-width="2"
@@ -71,6 +72,25 @@
                         <line x1="12" y1="3" x2="12" y2="15"></line>
                     </svg>
                 </button>
+                <button class="btn btn-ghost btn-icon btn-sm" id="btn-export" title="Export Profiles">
+                    <!-- Download icon -->
+                    <svg xmlns="http://www.w3.org/2000/svg" width="14" height="14" viewBox="0 0 24 24"
+                         fill="none" stroke="currentColor" stroke-width="2"
+                         stroke-linecap="round" stroke-linejoin="round">
+                        <path d="M21 15v4a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2v-4"></path>
+                        <polyline points="7 10 12 15 17 10"></polyline>
+                        <line x1="12" y1="15" x2="12" y2="3"></line>
+                    </svg>
+                </button>
+                <button class="btn btn-ghost btn-icon btn-sm" id="btn-recordings" title="Session Recordings">
+                    <!-- Film icon -->
+                    <svg xmlns="http://www.w3.org/2000/svg" width="14" height="14" viewBox="0 0 24 24"
+                         fill="none" stroke="currentColor" stroke-width="2"
+                         stroke-linecap="round" stroke-linejoin="round">
+                        <circle cx="12" cy="12" r="10"></circle>
+                        <circle cx="12" cy="12" r="3"></circle>
+                    </svg>
+                </button>
                 <button class="btn btn-ghost btn-icon btn-sm" id="btn-settings" title="Settings">
                     <!-- Cog icon -->
                     <svg xmlns="http://www.w3.org/2000/svg" width="14" height="14" viewBox="0 0 24 24"
@@ -177,6 +197,7 @@
     <script src="/src/js/settings.js"></script>
     <script src="/src/js/commands.js"></script>
     <script src="/src/js/secrets.js"></script>
+    <script src="/src/js/recordings.js"></script>
     <script src="/src/js/splitpane.js"></script>
     <script src="/src/js/app.js"></script>
 </body>