Cleanup: ConfirmModal primitive, drop dead PageStub, full-width editor (#107)

Three tidy-ups from the audit's deferred list:

- Extracted a ConfirmModal primitive (message + Cancel/Confirm, owning the
  busy lock + inline error) and folded the five hand-rolled confirm dialogs
  onto it: DeleteGarden, LeaveGarden, DeletePlant, DeleteSeedLot, ClearBed.
  Each is now just its message + mutation + labels. Bonus: ClearBed now
  shows a failure inline instead of swallowing it. (CopyGarden stays on
  Modal — it has a name field, not a plain confirm.)
- Removed components/PageStub.tsx — dead scaffolding, imported nowhere.
- The garden editor was squeezed into the max-w-5xl reading measure the
  other pages use; the canvas routes (editor + public garden) now go
  edge-to-edge on desktop, and the top bar matches so the brand aligns with
  the editor's left edge. Mobile was already full-width, so it's unchanged.

Verified live: the Delete-garden confirm renders/cancels; the desktop editor
now uses the full viewport width. tsc + vitest + build green.

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

web/src/components/gardens/LeaveGardenModal.tsx

@@ -1,8 +1,4 @@
-import { useState } from 'react'
-import { Modal } from '@/components/ui/Modal'
-import { Alert } from '@/components/ui/Alert'
-import { Button } from '@/components/ui/Button'
-import { errorMessage } from '@/lib/api'
+import { ConfirmModal } from '@/components/ui/ConfirmModal'
 import { useMe } from '@/lib/auth'
 import type { Garden } from '@/lib/gardens'
 import { useRemoveShare } from '@/lib/shares'
@@ -12,36 +8,22 @@ import { useRemoveShare } from '@/lib/shares'
 export function LeaveGardenModal({ garden, onClose }: { garden: Garden; onClose: () => void }) {
   const me = useMe()
   const remove = useRemoveShare(garden.id)
-  const [error, setError] = useState<string | null>(null)
-
-  async function onConfirm() {
-    if (!me.data) return
-    setError(null)
-    try {
-      await remove.mutateAsync(me.data.id)
-      onClose()
-    } catch (err) {
-      setError(errorMessage(err, 'Could not leave the garden.'))
-    }
-  }
-
   return (
-    <Modal title="Leave garden" onClose={onClose} busy={remove.isPending}>
-      <div className="flex flex-col gap-4">
-        <p className="text-sm text-muted">
-          Leave <span className="font-medium text-fg">{garden.name}</span>? You'll lose access until the owner
-          shares it with you again.
-        </p>
-        {error && <Alert>{error}</Alert>}
-        <div className="flex justify-end gap-2">
-          <Button type="button" variant="ghost" onClick={onClose} disabled={remove.isPending}>
-            Cancel
-          </Button>
-          <Button type="button" variant="danger" onClick={onConfirm} disabled={remove.isPending || !me.data}>
-            {remove.isPending ? 'Leaving…' : 'Leave'}
-          </Button>
-        </div>
-      </div>
-    </Modal>
+    <ConfirmModal
+      title="Leave garden"
+      confirmLabel="Leave"
+      busyLabel="Leaving…"
+      confirmDisabled={!me.data}
+      errorFallback="Could not leave the garden."
+      onConfirm={async () => {
+        if (me.data) await remove.mutateAsync(me.data.id)
+      }}
+      onClose={onClose}
+    >
+      <p className="text-sm text-muted">
+        Leave <span className="font-medium text-fg">{garden.name}</span>? You'll lose access until the owner
+        shares it with you again.
+      </p>
+    </ConfirmModal>
   )
 }