fix(input): an oversized paste no longer poisons the durable input queue (#484)

A single input over MAX_INPUT_LENGTH (64 KiB) was queued for reliable
delivery, refused by both transports (the WebSocket silently, POST with a
400), and never dropped: the client treated the 400 as transient, so the
frame was re-sent every 2 s forever, blocked every later input for that
session, and came back from localStorage on every reload.

- Client: a paste over the frame limit is split into in-limit frames
  (never cutting a surrogate pair) delivered in seq order; over 1 MiB, or
  an oversized mux write, it is refused with a toast and never queued.
- Client: the POST drain drops a frame answered 400/413; a WS error ACK
  drops it too; frames over the limit persisted by an older build are
  pruned on load.
- Server: the WebSocket answers an oversized sequenced frame with
  {t:'ia',seq,err:'too_large',max} instead of silence (an older client
  reads that as a plain ACK and drops it); the POST schema uses
  MAX_INPUT_LENGTH instead of a second 100000 limit.

Verified end to end on an isolated instance: a 110 KB paste reached the
PTY byte-identical over both the WebSocket and the POST path, a poisoned
120 KB persisted frame was pruned on load, and a 2 MB paste showed the
refusal toast with nothing queued.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/web/public/constants.js

@@ -762,6 +762,49 @@ function resolveTerminalFontWeights(settings) {
 // without a terminal, a clipboard, or a browser.
 // ---------------------------------------------------------------------------
 
+/**
+ * Largest single input frame the server accepts, in UTF-16 code units.
+ * ⚠️ Must equal MAX_INPUT_LENGTH in src/config/terminal-limits.ts (pinned by
+ * test/input-size-limit.test.ts). Both transports reject a longer frame, and
+ * before issue #484 the durable input queue retried such a frame forever.
+ */
+const INPUT_FRAME_MAX_CHARS = 64 * 1024;
+
+/**
+ * Largest paste the client will deliver at all. Anything up to this is split
+ * into INPUT_FRAME_MAX_CHARS frames that go out in seq order, so the PTY sees
+ * one contiguous byte stream (bracketed-paste markers included). Past it the
+ * input is refused with a toast rather than queued: every frame is persisted
+ * and retried until ACKed, so a multi-megabyte paste would pin the queue.
+ */
+const INPUT_PASTE_MAX_CHARS = 1024 * 1024;
+
+/**
+ * Split input into frames no longer than `max` code units, never cutting a
+ * surrogate pair in half (a lone surrogate reaches the PTY as U+FFFD).
+ *
+ * @param {string} data
+ * @param {number} [max]
+ * @returns {string[]}
+ */
+function splitInputFrames(data, max = INPUT_FRAME_MAX_CHARS) {
+  if (typeof data !== 'string' || data.length === 0) return [];
+  if (!(max >= 2)) max = 2;
+  if (data.length <= max) return [data];
+  const frames = [];
+  let start = 0;
+  while (start < data.length) {
+    let end = Math.min(start + max, data.length);
+    if (end < data.length) {
+      const code = data.charCodeAt(end - 1);
+      if (code >= 0xd800 && code <= 0xdbff) end--; // keep the pair together
+    }
+    frames.push(data.slice(start, end));
+    start = end;
+  }
+  return frames;
+}
+
 /**
  * Upper bound on an AUTO-copied selection.
  *
@@ -940,6 +983,11 @@ if (typeof window !== 'undefined') {
     compare: compareSessionActivity,
     sort: sortSessionsByActivity,
   };
+  window.CodemanInputLimit = {
+    FRAME_MAX_CHARS: INPUT_FRAME_MAX_CHARS,
+    PASTE_MAX_CHARS: INPUT_PASTE_MAX_CHARS,
+    split: splitInputFrames,
+  };
   window.CodemanAutoCopy = {
     decide: decideAutoCopy,
     MAX_CHARS: AUTO_COPY_MAX_CHARS,