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/layout/NotesDock.test.tsx

@@ -3,7 +3,7 @@ import { render, screen, fireEvent } from "@testing-library/react";
 import NotesDock from "./NotesDock";
 import type { Project, TerminalSession } from "../../lib/types";
 
-vi.mock("../notes/NotesPanel", () => ({
+vi.mock("../notes/NotesDockPanel", () => ({
   default: ({ projectId }: { projectId: string }) => (
     <div data-testid="panel">{`panel:${projectId}`}</div>
   ),

app/src/components/layout/NotesDock.tsx

@@ -7,7 +7,7 @@ import {
   NOTES_DOCK_MIN_WIDTH,
   NOTES_DOCK_MAX_WIDTH,
 } from "../../store/appState";
-import NotesPanel from "../notes/NotesPanel";
+import NotesDockPanel from "../notes/NotesDockPanel";
 import Button from "../ui/Button";
 
 /**
@@ -116,7 +116,7 @@ export default function NotesDock() {
       </div>
       <div className="flex-1 min-h-0">
         {projectId ? (
-          <NotesPanel projectId={projectId} />
+          <NotesDockPanel projectId={projectId} />
         ) : (
           <p className="p-4 text-[13px] text-[var(--text-secondary)]">
             Open a project or a terminal to see its notes.