fix: address review report across security, architecture, quality, tests

Implements the fixes from docs/REVIEW_REPORT.md (4-agent parallel review).
typecheck clean; 341 tests pass (16 files, +113); build:web ok; coverage
thresholds (80%) enforced in vitest.config.ts.

Critical:
- multi-device approval race: release held approval only when the last
  client detaches (closing one mirror no longer cancels another's prompt)
- unbounded session creation (DoS): Config.maxSessions cap (env MAX_SESSIONS),
  enforced in manager via the existing M4 exit(-1) path
- signal-handler leak: named SIGINT/SIGTERM/uncaughtException refs removed in close()
- terminal-session initialInput timer tracked + cleared on dispose
- tabs.addEntry null-as-cast type hole removed (build session before entry)

Should-fix:
- security-headers middleware + Origin/CSRF guard on DELETE /live-sessions[/:id]
- history.ts converted to fs/promises (async /sessions handler)
- removed dead clientDims map + blur protocol message end-to-end
- per-connection WS message rate limit (Config.maxMsgsPerSec)
- /sessions behavior kept; documented as accepted LAN risk (TECH_DOC §7)

Tests:
- new tmux / preview-grid / terminal-session (jsdom) / tabs (jsdom) suites
- extended history/config/manager/integration coverage incl. regressions

Hygiene:
- parsePositiveInt -> parseNonNegativeInt; ALLOWED_ORIGINS scheme validation
- log-injection sanitize; isLoopback handles 127.0.0.0/8 + IPv4-mapped
- operational constants moved into Config
- extracted public/preview-grid.ts (DRY launcher/manage)
- doc sweeps: ARCHITECTURE §8 runtime-handle exception, stale comments

test/protocol.test.ts

@@ -117,10 +117,9 @@ describe('parseClientMessage — valid messages', () => {
     if (r.ok) expect(r.message).toEqual({ type: 'reject' })
   })
 
-  it('parses blur (v0.4, no payload — withdraw size vote)', () => {
+  it('rejects blur (removed in the min→latest-writer pivot)', () => {
     const b = parseClientMessage(JSON.stringify({ type: 'blur' }))
-    expect(b.ok).toBe(true)
-    if (b.ok) expect(b.message).toEqual({ type: 'blur' })
+    expect(b.ok).toBe(false)
   })
 })