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/routes/file-routes.ts

@@ -239,7 +239,7 @@ export function registerFileRoutes(app: FastifyInstance, ctx: SessionPort): void
   // Serve raw file content (for images/binary files)
   app.get('/api/sessions/:id/file-raw', async (req, reply) => {
     const { id } = req.params as { id: string };
-    const { path: filePath } = req.query as { path?: string };
+    const { path: filePath, download } = req.query as { path?: string; download?: string };
     const session = findSessionOrFail(ctx, id);
 
     if (!filePath) {
@@ -292,6 +292,16 @@ export function registerFileRoutes(app: FastifyInstance, ctx: SessionPort): void
       };
 
       const content = await fs.readFile(resolvedPath);
+      if (download === 'true') {
+        const basename = filePath!.split('/').pop() || 'download';
+        reply.raw.writeHead(200, {
+          'Content-Type': mimeTypes[ext] || 'application/octet-stream',
+          'Content-Disposition': `attachment; filename="${basename}"`,
+          'Content-Length': content.length,
+        });
+        reply.raw.end(content);
+        return;
+      }
       reply.header('Content-Type', mimeTypes[ext] || 'application/octet-stream');
       reply.send(content);
     } catch (err) {