fix(security): CORS allowlist, URL scheme guard, state perms, ZIP slip, mirror docs

Phase 3 security audit follow-up. Medium and low-severity hardenings
plus a deferred-work plan for the cross-repo stream-token rollout.

Stream server CORS: replace the wildcard Access-Control-Allow-Origin
with an allowlist that echoes back only torrentclaw.com,
app.torrentclaw.com, the local Next dev port (3030 — matches the web
repo package.json) and any extras the operator adds via the new
downloads.cors_extra_origins TOML key. A Vary: Origin header is now
emitted whenever the request carries an Origin header so an
intermediate cache cannot serve a stale ACAO to a different origin.

URL scheme guard: openBrowser and OpenPlayer refuse any URL that is
not http(s). Combined with passing the URL after "--" wherever the
launched helper supports it (open, mpv, vlc, cvlc), this stops a
leading "-" from being parsed as a switch by the spawned process.

State file permissions: WriteState now writes 0o600 so the agent ID,
PID and counters cannot be enumerated by another local user on a
shared host. Matches the existing config file mode.

ZIP slip defense-in-depth: extractZip extracts the safety check into
safeZipPath, which canonicalises the entry name (normalising
backslashes to "/"), rejects "..", "../" prefix and "/../" interior
components, and verifies the final destination stays inside destDir
before opening any file.

Mirror fallback: documented the design for multi-provider
mirrors.json hosting in the comment block on DefaultStaticFallbackURLs
and added a follow-up note about signing it with the same ed25519
release key. The list is kept at one provider until the second host
is provisioned and added to torrentclaw-web's STATIC_FALLBACKS.

Deferred work: a new plan document Docs/plans/security-stream-token.md
covers the per-task stream token (Phase 2.2 of the original audit)
which requires coordinated web + CLI work and ships separately.

internal/cmd/daemon.go

@@ -241,6 +241,7 @@ func runDaemonStart() error {
 	// Create persistent stream server
 	streamSrv := engine.NewStreamServer(cfg.Download.StreamPort)
 	streamSrv.SetUPnPEnabled(cfg.Download.EnableUPnP)
+	streamSrv.SetCORSAllowedOrigins(cfg.Download.CORSExtraOrigins)
 	// Reap HLS tmpdirs left over from a previous daemon run before we start
 	// accepting new sessions. The in-memory registry doesn't survive a
 	// restart, so without this disk usage grows unbounded across restarts.