fix(ui): make long error toasts readable and copyable

A failed generation put the server's error straight into a toast. The
transformers "Unrecognized model" error is ~4.8KB, of which the first
sentence carries the meaning and the remaining 4.7KB is an alphabetical
list of every architecture it knows. In a 420px toast with
overflow-hidden and no scroll, that clipped the text at both ends and
pushed the close button off-screen: unreadable and undismissable.

- ToastDescription is capped at 40vh and scrolls, wraps on whitespace
  and breaks long unspaced tokens so a path cannot widen the toast.
- The toast aligns to the start rather than centring, so the title
  stays visible next to a tall description.
- condenseError() keeps the head of an oversized error, cutting at the
  first newline or the last sentence end inside a 400-char budget, and
  reports how many characters it dropped. Short errors pass through
  untouched.
- When it does truncate, the toast offers a Copy action for the full
  text and points at Settings -> Logs.

Verified against the real 4795-char error: 4795 -> 400 chars keeping
both meaningful sentences.

The hook moves to .tsx to render ToastAction, matching useAutoUpdater.tsx
which is a .tsx hook for the same reason. createElement was tried first
but this repo's ToastActionElement type is the older shadcn definition
(ReactElement<typeof ToastAction>) which only accepts JSX-constructed
elements.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

app/src/lib/utils/errorText.ts

@@ -0,0 +1,67 @@
+/**
+ * Condense a server error for display in a toast.
+ *
+ * Some backend errors are enormous and mostly noise. The transformers
+ * "Unrecognized model" error is ~4.8KB, of which the first sentence carries all
+ * the meaning and the remaining 4.7KB is an alphabetical list of every model
+ * architecture it knows about. Rendering that in a 420px toast clipped the text
+ * at both ends and pushed the close button off-screen.
+ *
+ * The rule is deliberately generic rather than pattern-matching any one
+ * library: keep the head, cut at the most natural boundary available inside the
+ * budget, and report how much was dropped so nobody assumes they read it all.
+ */
+
+/** Characters of an error worth putting in a toast. Roughly the first
+ * paragraph — enough for a sentence or two of real message. */
+const TOAST_ERROR_BUDGET = 400;
+
+/** Below this there is nothing to gain by condensing. */
+const MIN_TO_CONDENSE = TOAST_ERROR_BUDGET + 120;
+
+export interface CondensedError {
+  /** What to show in the toast. */
+  display: string;
+  /** The untouched original, for copying. */
+  full: string;
+  /** Whether `display` is shorter than `full`. */
+  truncated: boolean;
+  /** How many characters `display` leaves out. */
+  omitted: number;
+}
+
+export function condenseError(raw: string | null | undefined): CondensedError {
+  const full = (raw ?? '').trim();
+
+  if (full.length <= MIN_TO_CONDENSE) {
+    return { display: full, full, truncated: false, omitted: 0 };
+  }
+
+  // A traceback's first line is nearly always the message; prefer it whenever
+  // it fits, since a newline is a stronger boundary than any punctuation.
+  const firstLine = full.split('\n', 1)[0].trim();
+  let head =
+    firstLine.length > 0 && firstLine.length <= TOAST_ERROR_BUDGET
+      ? firstLine
+      : full.slice(0, TOAST_ERROR_BUDGET);
+
+  if (head.length < full.length && head === full.slice(0, head.length)) {
+    // Back off to the last sentence end inside the budget so the text does not
+    // stop mid-word. Only accept it if it keeps most of the budget — otherwise
+    // a stray early period would throw away usable context.
+    const lastStop = Math.max(head.lastIndexOf('. '), head.lastIndexOf('? '));
+    if (lastStop > TOAST_ERROR_BUDGET * 0.4) {
+      head = head.slice(0, lastStop + 1);
+    }
+  }
+
+  head = head.trimEnd();
+  const omitted = full.length - head.length;
+
+  // Guard against the boundary search having produced nothing shorter.
+  if (omitted <= 0) {
+    return { display: full, full, truncated: false, omitted: 0 };
+  }
+
+  return { display: `${head} …`, full, truncated: true, omitted };
+}