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

src/session/session.ts

@@ -108,7 +108,6 @@ export function createSession(
     meta,
     buffer: createRingBuffer(cfg.scrollbackBytes),
     clients: new Set(),
-    clientDims: new Map(),
     detachedAt: null,
     lastOutputAt: now,
     exitedAt: null,
@@ -161,7 +160,6 @@ export function attachWs(session: Session, ws: WebSocketLike): void {
  */
 export function detachWs(session: Session, ws: WebSocketLike, now: number): void {
   session.clients.delete(ws);
-  session.clientDims.delete(ws);
   // A client leaving does NOT resize the PTY — whatever device is still actively
   // viewing keeps its size. Start the idle clock only when the last client goes.
   if (session.clients.size === 0) {
@@ -175,36 +173,26 @@ export function writeInput(session: Session, data: string): void {
   session.pty.write(data);
 }
 
-/**
- * Record one client's requested dims and resize the PTY to the new min across
- * all clients (tmux-style). No-op after exit (L4); idempotent when unchanged.
- */
 /**
  * Latest-writer-wins: the device that most recently fit/focused drives the PTY
  * size, so whichever device you are actively using is full-screen. (A shared
  * PTY can only be one size; min-sizing letterboxed the bigger screen.) The
  * frontend re-sends dims when a pane is shown or its window regains focus, so
  * switching devices reclaims full size. No-op after exit (L4) / when unchanged.
+ *
+ * The `ws` argument identifies the requesting client; the size is applied
+ * directly to the PTY (no per-client dims map — latest writer wins).
  */
 export function setClientDims(
   session: Session,
-  ws: WebSocketLike,
+  _ws: WebSocketLike,
   cols: number,
   rows: number,
 ): void {
   if (session.exitedAt !== null) return;
-  session.clientDims.set(ws, { cols, rows });
   applyDims(session, cols, rows);
 }
 
-/**
- * Forget a client's size when its tab goes hidden — but do NOT resize: the
- * device still actively viewing keeps its size (no letterboxing of a mirror).
- */
-export function clearClientDims(session: Session, ws: WebSocketLike): void {
-  session.clientDims.delete(ws);
-}
-
 /**
  * Kill the session (idle reclaim). Under tmux this ends the actual shell via
  * `tmux kill-session` (H1); killing only the client pty would leave the tmux