feat(terminal): add reconnection buffer support for persistent sessions (#14377)

* feat(terminal): add reconnection buffer support for persistent sessions

Fix two related issues:
1. Reconnecting to persistent sessions shows blank screen - server now
   automatically sends historical buffer on reconnection via SessionState
   machine with pending_buffer, eliminating the need for client-initiated
   buffer requests.
2. Terminal output before view ready causes NaN errors - buffer output
   chunks on client side until terminal view has valid dimensions, then
   flush in order on first valid resize.

Rust side:
- Introduce SessionState enum (Closed/Active) replacing bool is_opened
- Auto-attach pending buffer on reconnection in handle_open()
- Always drain output channel in read_outputs() to prevent overflow
- Increase channel buffer from 100 to 500
- Optimize get_recent() to collect whole chunks (avoids ANSI truncation)
- Extract create_terminal_data_response() helper (DRY)
- Add reconnected flag to TerminalOpened protobuf message

Flutter side:
- Buffer output chunks until terminal view has valid dimensions
- Flush buffered output on first valid resize via _markViewReady()
- Clear terminal on reconnection to avoid duplicate output from buffer replay
- Fix max_bytes type (u32) to match protobuf definition
- Pass reconnected field through FlutterHandler event

Signed-off-by: fufesou <linlong1266@gmail.com>

* fix(terminal): add two-phase SIGWINCH for TUI app redraw and session remap on reconnection

Fix TUI apps (top, htop) not redrawing after reconnection. A single
resize-then-restore is too fast for ncurses to detect a size change,
so split across two read_outputs() polling cycles (~30ms apart) to
force a full redraw.

Also fix reconnection failure when client terminal_id doesn't match
any surviving server-side session ID by remapping the lowest surviving
session to the requested ID.

Rust side:
- Add two-phase SIGWINCH state machine (SigwinchPhase: TempResize →
  Restore → Idle) with retry logic (max 3 attempts per phase)
- Add do_sigwinch_resize() for cross-platform PTY resize (direct PTY
  and Windows helper mode)
- Add session remap logic for non-contiguous terminal_id reconnection
- Extract try_send_output() helper with rate-limited drop logging (DRY)
- Add 3-byte limit to UTF-8 continuation byte skipping in get_recent()
  to prevent runaway on non-UTF-8 binary data
- Remove reconnected flag from flutter.rs (unused on client side)

Flutter side:
- Add reconnection screen clear and deferred flush logic
- Filter self from persistent_sessions restore list
- Add comments for web-related changes

Signed-off-by: fufesou <linlong1266@gmail.com>

---------

Signed-off-by: fufesou <linlong1266@gmail.com>

flutter/lib/models/terminal_model.dart

@@ -266,8 +266,8 @@ class TerminalModel with ChangeNotifier {
 
   void _handleTerminalOpened(Map<String, dynamic> evt) {
     final bool success = getSuccessFromEvt(evt);
-    final String message = evt['message'] ?? '';
-    final String? serviceId = evt['service_id'];
+    final String message = evt['message']?.toString() ?? '';
+    final String? serviceId = evt['service_id']?.toString();
 
     debugPrint(
         '[TerminalModel] Terminal opened response: success=$success, message=$message, service_id=$serviceId');
@@ -275,7 +275,18 @@ class TerminalModel with ChangeNotifier {
     if (success) {
       _terminalOpened = true;
 
-      // Service ID is now saved on the Rust side in handle_terminal_response
+      // On reconnect ("Reconnected to existing terminal"), server may replay recent output.
+      // If this TerminalView instance is reused (not rebuilt), duplicate lines can appear.
+      // We intentionally accept this tradeoff for now to keep logic simple.
+
+      // Fallback: if terminal view is not yet ready but already has valid
+      // dimensions (e.g. layout completed before open response arrived),
+      // mark view ready now to avoid output stuck in buffer indefinitely.
+      if (!_terminalViewReady &&
+          terminal.viewWidth > 0 &&
+          terminal.viewHeight > 0) {
+        _markViewReady();
+      }
 
       // Process any buffered input
       _processBufferedInputAsync().then((_) {
@@ -358,8 +369,7 @@ class TerminalModel with ChangeNotifier {
       // because it only affects the pre-layout buffering window and the
       // terminal will self-correct on subsequent output.
       if (text.length >= _kMaxOutputBufferChars) {
-        final truncated =
-            text.substring(text.length - _kMaxOutputBufferChars);
+        final truncated = text.substring(text.length - _kMaxOutputBufferChars);
         _pendingOutputChunks
           ..clear()
           ..add(truncated);