Address editor-mode review: mode↔focus↔rail syncing

Gadfly on #99, the real ones — all in the mode/focus/rail interplay:

- (3 models) Closing a journal/assistant rail while a bed was focused
  hard-set mode='fixtures', docking the OBJECT palette inside the focused
  bed with the seed tray unreachable. Derive it: closing a panel returns
  to Plants if still focused, else Fixtures. The canvas-mode effect now
  also follows UN-focus (plants→fixtures) and won't override a panel mode.
- (2 models) Plants mode on a focused non-plantable object (reachable via
  a ?focus= deep link) showed the misleading "tap a bed" hint and no way
  out on mobile. Now it says what's wrong and offers Done (exit focus).
- Selecting an object leaves a panel mode, so closing the inspector can't
  strand the bar on Journal/Assistant with nothing open.
- Tapping Fixtures steps out of a focused bed (you're arranging again).
- Viewer mode bar drops Fixtures/Plants (a viewer can't place anything),
  leaving Journal + Assistant.
- Safety: if the assistant capability flips off live while it's the active
  mode, fall back to a canvas mode and close the orphaned chat rail.

Maintainability: extracted the shared seed-tray + Done + Clear cluster into
PlantPlacementTools (was duplicated between the desktop focus toolbar and
the mobile Plants strip); DEFAULT_MODE const replaces the twice-hardcoded
'fixtures'; selectMode reads the reactive railTab, not getState().

Verified live at 390px: focus a bed → Plants; open journal → close → back
to Plants (tray), not the Fixtures palette; tap Fixtures → exits focus.

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

web/src/editor/store.ts

@@ -18,6 +18,9 @@ export const MAX_SCALE = 20 // px per cm — fully zoomed in
 // side-column layout and treats this as a lighter hint.
 export type EditorMode = 'fixtures' | 'plants' | 'journal' | 'assistant'
 
+// Where the editor starts, and where it returns on a garden switch.
+export const DEFAULT_MODE: EditorMode = 'fixtures'
+
 interface EditorState {
   viewport: Viewport
   setViewport: (next: Viewport | ((prev: Viewport) => Viewport)) => void
@@ -94,7 +97,7 @@ export const useEditorStore = create<EditorState>((set) => ({
   viewport: { tx: 0, ty: 0, scale: 1 },
   setViewport: (next) => set((s) => ({ viewport: typeof next === 'function' ? next(s.viewport) : next })),
 
-  mode: 'fixtures',
+  mode: DEFAULT_MODE,
   setMode: (mode) => set({ mode }),
 
   selectedId: null,
@@ -144,6 +147,6 @@ export const useEditorStore = create<EditorState>((set) => ({
       railTab: null,
       seasonYear: null,
       journalObjectId: null,
-      mode: 'fixtures',
+      mode: DEFAULT_MODE,
     }),
 }))