Give the dock its own compact notes layout

The dock was showing `NotesPanel`, which is a master/detail layout: a column
of titles beside an editor. The previous commit made that survive dock width;
it did not make it right. At 352px the layout still spends roughly 356px of
height on chrome — dock header, panel header, title strip, a button row that
wraps, and a paragraph of help — before the body gets a pixel.

So the dock now shows one note. The title field names what is open and the
chevron beside it switches; New and Delete move into the overflow menu; the
help text goes. Chrome drops to about 112px and the body takes the rest.

The two surfaces are now different components, which contradicts a docstring
I wrote — "shared so the two cannot drift into different behaviour". That
claim was about behaviour, and behaviour was never in the layout: it is in
`useNotes` for the cache and its write ordering, and now in `useNoteDraft`,
extracted here so when a keystroke becomes a save is defined in exactly one
place. Only the layout diverges. `NotesPanel.shared.test.tsx` gets stronger
for it — it now mounts the dock panel and the tab panel together, which is
what the app actually does, instead of the same component twice.

`NoteSwitcher` is not `OverflowMenu` despite the shape being close: that keys
items by label, and notes are addressed by id, so two untitled notes — the
ordinary case — would collapse into one row. It is also not a `combobox`; an
input plus a listbox button is two honest controls, where the role would owe
active-descendant tracking and filtering that nothing here needs.

`SendToAgentButton` picks up `useUnavailable` from #49, which is what its
`disabled` plus explanatory `title` was already asking for. Four tests moved
from `toBeDisabled()` to the new contract, and one of them — "does nothing for
an empty note" — turned out never to have asserted that it does nothing. It
does now, for click and for Enter, which is the guard the swap needs.

It also gains `dropUp`, and that is load-bearing rather than cosmetic: the
dock clips its own overflow, so a session menu opening downward from a button
on the bottom edge is drawn outside the panel and never seen.

744 tests pass, 62 files. As before, jsdom has no layout engine: that the dock
now reads as compact is not something the suite can tell you.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011YPqHpjV4EL6RNEwrRKqQm

app/src/components/notes/NoteSwitcher.tsx

@@ -0,0 +1,112 @@
+import { useEffect, useRef, useState } from "react";
+import type { Note } from "../../lib/types";
+
+export const UNTITLED = "Untitled note";
+
+interface Props {
+  notes: Note[];
+  selectedId: string;
+  title: string;
+  onTitleChange: (value: string) => void;
+  onCommit: () => void;
+  onSelect: (id: string) => void;
+}
+
+/**
+ * One row that both names the current note and switches to another.
+ *
+ * The dock has no room for a permanent list of titles, so the title field
+ * doubles as the label of what is open and the chevron beside it holds the
+ * rest. Renaming therefore needs no separate affordance.
+ *
+ * Two honest controls rather than one `role="combobox"`: a text field and a
+ * button that opens a listbox. A real combobox owes its listbox keyboard
+ * navigation, active-descendant tracking and an input that filters — none of
+ * which this needs, and half of which is worse than not claiming the role.
+ *
+ * `OverflowMenu` is deliberately not reused here despite the shape being
+ * close. It keys its items by label, and notes are addressed by id: two
+ * untitled notes are the ordinary case and would collapse into one row.
+ */
+export default function NoteSwitcher({
+  notes,
+  selectedId,
+  title,
+  onTitleChange,
+  onCommit,
+  onSelect,
+}: Props) {
+  const [open, setOpen] = useState(false);
+  const rootRef = useRef<HTMLDivElement>(null);
+
+  // Same dismissal contract as `OverflowMenu`, so the two feel identical.
+  useEffect(() => {
+    if (!open) return;
+    const onDocClick = (e: MouseEvent) => {
+      if (!rootRef.current?.contains(e.target as Node)) setOpen(false);
+    };
+    const onKey = (e: KeyboardEvent) => {
+      if (e.key === "Escape") setOpen(false);
+    };
+    document.addEventListener("mousedown", onDocClick);
+    document.addEventListener("keydown", onKey);
+    return () => {
+      document.removeEventListener("mousedown", onDocClick);
+      document.removeEventListener("keydown", onKey);
+    };
+  }, [open]);
+
+  return (
+    <div ref={rootRef} className="relative flex items-center gap-1 min-w-0">
+      <input
+        value={title}
+        onChange={(e) => onTitleChange(e.target.value)}
+        onBlur={onCommit}
+        placeholder="Note title"
+        aria-label="Note title"
+        className="flex-1 min-w-0 px-2 h-7 bg-[var(--bg-primary)] border border-[var(--border-color)] rounded-[var(--radius-control)] text-[13px] text-[var(--text-primary)] focus:border-[var(--accent)] transition-colors"
+      />
+      <button
+        type="button"
+        aria-label="Switch note"
+        aria-haspopup="listbox"
+        aria-expanded={open}
+        onClick={() => setOpen((o) => !o)}
+        className="inline-flex items-center justify-center h-7 w-6 flex-shrink-0 rounded-[var(--radius-control)] border border-[var(--border-color)] bg-[var(--bg-tertiary)] text-[var(--text-secondary)] hover:text-[var(--text-primary)] hover:bg-[var(--border-color)] transition-colors"
+      >
+        <span aria-hidden="true" className="leading-none text-[10px]">▾</span>
+      </button>
+      {open && (
+        <div
+          role="listbox"
+          aria-label="Notes"
+          className="absolute right-0 top-full mt-1 z-40 w-full max-h-64 overflow-y-auto py-1 bg-[var(--bg-overlay)] border border-[var(--border-color)] rounded-[var(--radius-panel)]"
+          style={{ boxShadow: "var(--shadow-overlay)" }}
+        >
+          {/* Buttons directly inside the listbox: wrapping each in an `<li>`
+              would put an implicit `listitem` between the listbox and its
+              options, which is not a child role a listbox owns. */}
+          {notes.map((n) => (
+            <button
+              key={n.id}
+              type="button"
+              role="option"
+              aria-selected={n.id === selectedId}
+              onClick={() => {
+                onSelect(n.id);
+                setOpen(false);
+              }}
+              className={`block w-full text-left px-3 py-1.5 text-xs truncate transition-colors hover:bg-[var(--bg-tertiary)] ${
+                n.id === selectedId
+                  ? "text-[var(--text-primary)] bg-[var(--bg-tertiary)]"
+                  : "text-[var(--text-secondary)]"
+              }`}
+            >
+              {n.title.trim() || UNTITLED}
+            </button>
+          ))}
+        </div>
+      )}
+    </div>
+  );
+}