feat: cherry-pick keyboard UX and file download from community PRs

Cherry-picked from PR #60 (keyboard UX) and PR #61 (file download):

- Alt+1-9 session switching
- Disable Ctrl+K (too easy to trigger accidentally)
- Session rename with prefix preservation (w1-case: description)
- Shift+Enter / Ctrl+Enter multiline input via tmux send-keys -H
- Android virtual keyboard fix for non-composition input
- File download button in browser file explorer (?download=true)

Dropped from PR #60: stale package-lock.json, upload popup (missing upload.html)
Dropped from PR #61: standalone /api/download endpoint (arbitrary fs access)
Fixed from PR #60: execFileSync replaced with async execFile

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/web/public/styles.css

@@ -330,6 +330,15 @@ body {
   text-overflow: ellipsis;
 }
 
+.session-tab .tab-prefix {
+  color: var(--text-muted);
+}
+
+.session-tab .tab-suffix {
+  color: var(--text);
+  font-weight: 500;
+}
+
 /* Tab folder path — hidden by default, shown via .tabs-show-folder on container */
 .session-tab .tab-folder {
   font-size: 0.6rem;
@@ -6579,6 +6588,22 @@ kbd {
   padding-left: 0.5rem;
 }
 
+.file-tree-download {
+  display: none;
+  margin-left: auto;
+  padding: 0 4px;
+  color: #888;
+  text-decoration: none;
+  font-size: 12px;
+  flex-shrink: 0;
+}
+.file-tree-item:hover .file-tree-download {
+  display: inline;
+}
+.file-tree-download:hover {
+  color: #4fc3f7;
+}
+
 /* File tree indent levels */
 .file-tree-item[data-depth="0"] { padding-left: 0.5rem; }
 .file-tree-item[data-depth="1"] { padding-left: 1.25rem; }