web: fix red-team breaks in restore, drop-open, and plot paths

Three attack agents (state-machine tracing, executable input fuzzing,
browser-API review) found two breaks and several degradations; all are
fixed here, with the fuzzer's 22 tests kept as regression suites.

Breaks:
- Untitled tabs (and dot-names like ".dxf") autosave as DXF, but the
  restore name defaulted to .dwg — the worker dispatches its reader by
  the name's extension, so the only copy of unsaved work could never
  be restored and was re-offered forever. recovered_name now uses the
  worker's rsplit('.') convention and defaults extensionless names to
  dxf, matching new-document serialization.
- " (recovered)" stacked one suffix per crash-restore cycle; the
  marker is now stripped idempotently before appending.
- Opening a file (Ctrl+O / OPEN / automation) while a web open was in
  flight clobbered it: the in-flight open's bytes were lost and every
  queued restore prompt stalled for the session. on_open_file now
  refuses while an open is running (web only).

Hardening:
- Escape on the Recover Drawing modal defers instead of deleting the
  only snapshot; the Discard button still removes it.
- A cap-rejected restore enqueue warns in-app and re-offers the next
  snapshot; the drop queue cap rises to 32.
- Exiting mid-round no longer races snapshot removals (deferrals are
  drained, the sweep skips while a round is in flight); closing a tab
  with "Don't save" now also removes its web snapshot; the snapshot
  key derivation is shared via web_autosave_key.
- The plot-preview overlay no longer leaks a full PDF copy per preview
  (the Download closure lives in a replaceable registry).
- A corrupt autosave index.json fails the round/boot instead of
  silently resetting every row; plot pages with non-finite or non-
  positive sheet dimensions error instead of emitting a NaN MediaBox.
- The drop filter accepts dot-less "DWG"/"dxf" names the worker
  parses (same rsplit convention on both gates).

(cherry picked from commit 3eab00bfbf)

Ported onto main (#1717 review): the five standalone tests/*_redteam.rs
files stay out — no test target builds them and they exercised copies of
the functions rather than the code itself.

src/io/web_drop.rs

@@ -12,7 +12,8 @@ use wasm_bindgen_futures::JsFuture;
 
 /// A stray multi-drop must not grow the queue without bound; extras are
 /// rejected with a console warning instead of being silently dropped.
-const QUEUE_CAP: usize = 8;
+/// Entries are just name+bytes; 32 drawings is a realistic folder drop.
+const QUEUE_CAP: usize = 32;
 
 /// One queued open. `restore_key` tags an autosave-restore file so its
 /// snapshot is deleted only when THIS open succeeds — matching by identity,
@@ -41,7 +42,9 @@ fn lock_queue() -> std::sync::MutexGuard<'static, VecDeque<QueuedOpen>> {
         .unwrap_or_else(|poisoned| poisoned.into_inner())
 }
 
-fn push(entry: QueuedOpen) {
+/// Returns `false` (after a console warning) when the queue is full and the
+/// entry was NOT stored — the caller decides how to surface the loss.
+fn push(entry: QueuedOpen) -> bool {
     let name = entry.name.clone();
     let mut queue = lock_queue();
     if queue.len() >= QUEUE_CAP {
@@ -49,35 +52,40 @@ fn push(entry: QueuedOpen) {
         warn(&format!(
             "ocs: dropped-file queue full; ignoring \"{name}\""
         ));
-        return;
+        return false;
     }
     queue.push_back(entry);
+    true
 }
 
 /// Queue already-read drawing bytes (drag-and-drop) for the 50 ms poll to
-/// open through the guarded web-picker flow.
-pub fn enqueue_open(name: String, bytes: Vec<u8>) {
+/// open through the guarded web-picker flow. Returns `false` when the queue
+/// is full and the bytes were dropped.
+pub fn enqueue_open(name: String, bytes: Vec<u8>) -> bool {
     push(QueuedOpen {
         name,
         bytes,
         restore_key: None,
-    });
+    })
 }
 
-/// Queue an autosave-restore file tagged with its snapshot key.
-pub fn enqueue_restore(name: String, bytes: Vec<u8>, restore_key: String) {
+/// Queue an autosave-restore file tagged with its snapshot key. Returns
+/// `false` when the queue is full and the bytes were dropped (the OPFS
+/// snapshot survives; the caller must not wait for an open that will
+/// never start).
+pub fn enqueue_restore(name: String, bytes: Vec<u8>, restore_key: String) -> bool {
     push(QueuedOpen {
         name,
         bytes,
         restore_key: Some(restore_key),
-    });
+    })
 }
 
 /// JS-callable entry (same shape as `ocs_control_submit`) for hosts that
 /// forward drops themselves.
 #[wasm_bindgen::prelude::wasm_bindgen]
 pub fn ocs_web_drop_file(name: String, bytes: js_sys::Uint8Array) {
-    enqueue_open(name, bytes.to_vec());
+    let _ = enqueue_open(name, bytes.to_vec());
 }
 
 /// Pop the oldest dropped file that the poll has not started opening yet.
@@ -179,7 +187,10 @@ pub fn install() {
                 for (name, read) in reads {
                     match JsFuture::from(read).await {
                         Ok(buffer) => {
-                            enqueue_open(name, js_sys::Uint8Array::new(&buffer).to_vec())
+                            // A full queue warns on the console; the drop
+                            // handler has no command line to talk to.
+                            let _ =
+                                enqueue_open(name, js_sys::Uint8Array::new(&buffer).to_vec());
                         }
                         Err(error) => {
                             web_sys::console::error_1(&wasm_bindgen::JsValue::from_str(&format!(
@@ -203,7 +214,11 @@ pub fn install() {
 }
 
 fn is_drawing_name(name: &str) -> bool {
-    name.rsplit_once('.').is_some_and(|(_, extension)| {
-        matches!(extension.to_ascii_lowercase().as_str(), "dwg" | "dxf")
-    })
+    // Same convention as the worker's reader dispatch (`rsplit('.').next()`):
+    // a dot-less name is its own extension, so a file literally named
+    // "DWG"/"dxf" parses and must not be filtered out here.
+    matches!(
+        name.rsplit('.').next().unwrap_or_default().to_ascii_lowercase().as_str(),
+        "dwg" | "dxf"
+    )
 }