Chat panel in the garden editor (#57)

The conversational surface, in the editor rather than on its own page. The
reason is the feedback loop: watching the canvas change as the agent works IS
the confirmation, which is exactly what makes "act freely without asking first"
tolerable. It also means the agent never has to guess which garden you mean.

Tool calls surface as they happen, in the app's own vocabulary — "Clearing a
bed", "Looking up a plant" — not raw tool names or JSON. That is the difference
between the panel feeling like it's doing something and feeling like it's hung,
which matters because a replant makes a dozen calls over tens of seconds. An
unknown tool degrades to readable words rather than showing snake_case at the
user, so the client can lag the server by a tool without looking broken.

The canvas refreshes as each step lands, not just at the end. Refreshing only on
completion would put the whole point of siting the chat here — watching it work
— behind the same wait that streaming exists to remove.

Undo sits on the turn itself, so the common case never involves opening the
History panel. It uses #49's useUndo, not a second implementation, which meant
giving that hook an UndoTarget: the chat knows a turn's change set id but not
its tally, and fabricating counts to satisfy the type would have produced a
confidently wrong "1 of 1 changes undone". describeUndo now says "Partly undone"
when it has no denominator rather than inventing one.

The panel is only offered when the instance actually has the assistant
configured, via a new /capabilities read. The routes 404 without a key, so
without this the client would have to probe for a 404 to find out — and a tab
that opens onto an apology is worse than no tab.

Streaming is hand-rolled over fetch rather than EventSource, which can only
issue GETs and this needs a POST body. The wire format is still SSE, so a proxy
that understands it doesn't buffer and the server wouldn't change if EventSource
became viable. Partial frames are buffered across chunks and a malformed frame
is skipped rather than killing a working stream.

Errors read as sentences, and as different sentences: a permission refusal, a
timeout and a model failure want different reactions. Every failure path
refreshes, because something may have landed before it failed — and says where
to look for it.

Closes #57

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

DESIGN.md

@@ -69,6 +69,7 @@ GET,POST /gardens/:id/journal PATCH,DELETE /journal/:id           (editor writes
 GET  /gardens/:id/journal/counts  ← entries per object, for the "has notes" indicator
 POST /agent/chat              ← SSE: step events, then the finished turn (editor only)
 GET,DELETE /gardens/:id/agent/history                             (the actor's own thread)
+GET  /capabilities            ← what this instance can do, so the UI offers only what works
 GET,POST /gardens/:id/shares  PATCH,DELETE /gardens/:id/shares/:userId    (invite by email)
 ```
 
@@ -118,7 +119,7 @@ React 19 + TypeScript + Vite + Tailwind 4 (`@tailwindcss/vite`), `@tanstack/reac
 - **Routes:** `/login`, `/register`, `/gardens` (list), `/gardens/:id` (editor, `?focus=objectId`), `/plants` (catalog). Auth guard on the router root via `/auth/me`.
 - **State:** TanStack Query for all server state (editor keyed on `gardens/:id/full`; optimistic mutations with version-conflict rollback). One small Zustand store for ephemeral editor state only: viewport, selection, focused object, active tool, in-flight drag.
 - **Editor components (`web/src/editor/`):** `GardenCanvas` (svg root + viewport g), `useViewport` (use-gesture pan/zoom/pinch), `ObjectShape`, `PlopMarker` (semantic-zoom branching), `Palette` (drag-to-place object kinds), `EditorRail` (the one side panel), `Inspector`, `HistoryPanel`, `PlantPicker`.
-- **One rail, tabs inside it.** The inspector, history, journal, and later the chat panel all want the same strip of screen; rather than each bolting on its own chrome they are tabs in `EditorRail` — so the canvas is one width instead of a different width per panel, and adding a panel is adding a tab. Selecting an object switches to the Inspector tab automatically, so the rail is never something you operate before you can edit; on a phone the same tabs render in the bottom sheet the inspector already used. Pure geometry helpers (local↔world transforms, unit formatting) in `web/src/lib/geometry.ts`, unit-tested.
+- **One rail, tabs inside it.** The inspector, history, journal and assistant all want the same strip of screen; rather than each bolting on its own chrome they are tabs in `EditorRail` — so the canvas is one width instead of a different width per panel, and adding a panel is adding a tab. Selecting an object switches to the Inspector tab automatically, so the rail is never something you operate before you can edit; on a phone the same tabs render in the bottom sheet the inspector already used. Pure geometry helpers (local↔world transforms, unit formatting) in `web/src/lib/geometry.ts`, unit-tested.
 
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