fix: security hardening and cleanup from community PR cherry-picks

- Add HTML sanitizer for markdown rendering (XSS prevention)
- Switch service worker to network-first caching (deploys take effect immediately)
- Sanitize Content-Disposition filenames (header injection prevention)
- Expose session.muxName getter, replace unsafe `as any` cast
- Static import for execFile, update CLAUDE.md keyboard shortcuts

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

src/web/routes/file-routes.ts

@@ -293,7 +293,9 @@ export function registerFileRoutes(app: FastifyInstance, ctx: SessionPort): void
 
       const content = await fs.readFile(resolvedPath);
       if (download === 'true') {
-        const basename = filePath!.split('/').pop() || 'download';
+        const rawBasename = filePath!.split('/').pop() || 'download';
+        // Sanitize filename for Content-Disposition header (prevent header injection)
+        const basename = rawBasename.replace(/["\\\r\n]/g, '_');
         reply.raw.writeHead(200, {
           'Content-Type': mimeTypes[ext] || 'application/octet-stream',
           'Content-Disposition': `attachment; filename="${basename}"`,

src/web/routes/session-routes.ts

@@ -7,6 +7,7 @@
 import { FastifyInstance } from 'fastify';
 import { join, dirname } from 'node:path';
 import { existsSync, statSync, mkdirSync, writeFileSync } from 'node:fs';
+import { execFile } from 'node:child_process';
 import fs from 'node:fs/promises';
 import {
   ApiErrorCode,
@@ -545,13 +546,12 @@ export function registerSessionRoutes(
     }
 
     const session = findSessionOrFail(ctx, id);
-    const muxName = (session as any)._muxSession?.muxName;
+    const muxName = session.muxName;
     if (!muxName) {
       return createErrorResponse(ApiErrorCode.INVALID_INPUT, 'No tmux session');
     }
 
     try {
-      const { execFile } = await import('child_process');
       await new Promise<void>((resolve, reject) => {
         execFile('tmux', ['send-keys', '-H', '-t', muxName, ...hex], { timeout: 5000 }, (err) => {
           if (err) reject(err);