Rough edges: mobile 100dvh, session-expiry in chat, sticky error toasts, journal date filter (#85)

The safe, self-contained wins from the #85 bundle:

- **Mobile viewport.** The editor used `100vh`, which on mobile Safari/Chrome is
  the LARGEST viewport (URL bar hidden), so with the bar visible the canvas
  bottom and Fit button were pushed under the browser chrome. `100dvh` fixes it.
- **Session expiry mid-chat.** A 401 during a chat turn was mapped to "the
  assistant is not available right now" — sending the user to debug a config
  problem that isn't there. Now it says the session expired and redirects to
  /login, preserving the path, like the rest of the app treats 401.
- **Error toasts persist.** Error toasts were the primary report that a mutation
  failed, yet auto-dismissed at 4s with no way to retrieve them — look away and
  it's gone. Errors now stay until dismissed; info toasts still time out; both
  get a close button.
- **Journal date filter.** `from`/`to` existed in the API and JournalFilter but
  had no UI, so "show me last spring" was unreachable. Added two date inputs
  (with a Clear) that feed the existing filter. No backend change.

Deferred to their own follow-ups (too big for this bundle, or need a decision):
revision-history retention/pruning, the agent toolbox's missing corrective tools,
plop-level journal entries from the UI, and the editor's max-width cap (a layout
call worth confirming rather than changing blind). The DESIGN.md API-listing
drift the issue mentioned was already fixed in #89.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01H3zbym8Doka2d7D48maSgZ

web/src/components/ui/toast.tsx

@@ -32,21 +32,34 @@ export const toast = {
 // Param is `item`, not `toast`, so it doesn't shadow the module's `toast` export.
 function ToastItem({ item }: { item: Toast }) {
   const dismiss = useToastStore((s) => s.dismiss)
+  const isError = item.tone === 'error'
   useEffect(() => {
+    // Error toasts are the primary report that a mutation failed, so they do NOT
+    // auto-dismiss — a user who looked away at second 4 would otherwise lose the
+    // only notice, with nothing to retrieve (#85). Info toasts still time out.
+    if (isError) return
     const t = setTimeout(() => dismiss(item.id), 4000)
     return () => clearTimeout(t)
-  }, [item.id, dismiss])
+  }, [item.id, dismiss, isError])
   return (
     <div
-      role={item.tone === 'error' ? 'alert' : 'status'}
+      role={isError ? 'alert' : 'status'}
       className={cn(
-        'pointer-events-auto rounded-md border px-3 py-2 text-sm shadow-md',
-        item.tone === 'error'
+        'pointer-events-auto flex items-start gap-2 rounded-md border px-3 py-2 text-sm shadow-md',
+        isError
           ? 'border-red-500/40 bg-red-500/10 text-red-700 dark:text-red-300'
           : 'border-border bg-surface text-fg',
       )}
     >
-      {item.message}
+      <span className="flex-1">{item.message}</span>
+      <button
+        type="button"
+        onClick={() => dismiss(item.id)}
+        aria-label="Dismiss"
+        className="-mr-1 shrink-0 rounded px-1 text-current opacity-60 outline-none hover:opacity-100 focus-visible:ring-2 focus-visible:ring-current/40"
+      >
+        ✕
+      </button>
     </div>
   )
 }