Add gardens UI: list page + create/edit/delete (#8)

The /gardens page is now home base, backed by the #7 API.

- lib/units.ts: cm <-> meters/feet conversion + formatCm/formatDimensions
  (metric "m", imperial "ft/in"). Minimal for #8; #9's geometry lib
  consolidates.
- lib/gardens.ts: zod gardenSchema + useGardens/useCreateGarden/
  useUpdateGarden/useDeleteGarden (mutations invalidate the list) and
  conflictGarden() to pull the fresh row out of a 409.
- GardensPage: loading/empty/error states; empty state prompts a first
  garden; responsive card grid (single column on phones).
- GardenCard: name, dimensions formatted per the garden's unit, notes
  preview; body links into /gardens/:id, edit/delete kept out of the link.
- GardenFormModal: create/edit with a unit selector; dimensions are entered
  in the chosen unit and converted to cm (switching units converts the
  current values). A 409 rebases the form onto the server's fresh row.
- DeleteGardenModal: confirmation before removing.
- ui/: Modal (Escape/backdrop close), TextArea, Select.

Verified in a real browser against the embedded binary: create appears
without reload; edit persists (version 2), form prefilled from stored cm
converted back to the garden's unit; delete confirms then removes -> empty
state; an imperial 4 ft x 8 ft garden stores 122 x 244 cm and shows
"4' 0" x 8' 0"". tsc --noEmit clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01JdQpdYYsTgtkJBxbcpAszi

web/src/components/gardens/GardenFormModal.tsx

@@ -0,0 +1,156 @@
+import { useState, type FormEvent } from 'react'
+import { Modal } from '@/components/ui/Modal'
+import { Alert } from '@/components/ui/Alert'
+import { Button } from '@/components/ui/Button'
+import { Select } from '@/components/ui/Select'
+import { TextArea } from '@/components/ui/TextArea'
+import { TextField } from '@/components/ui/TextField'
+import { errorMessage } from '@/lib/api'
+import { conflictGarden, useCreateGarden, useUpdateGarden, type Garden } from '@/lib/gardens'
+import { cmFromDisplay, dimensionUnitLabel, displayFromCm, type UnitPref } from '@/lib/units'
+
+const DEFAULT_METERS = 10 // matches the server's 10 m default
+
+const unitOptions = [
+  { value: 'metric', label: 'Metric (m)' },
+  { value: 'imperial', label: 'Imperial (ft)' },
+]
+
+function dimString(cm: number | undefined, unit: UnitPref): string {
+  return cm === undefined ? String(DEFAULT_METERS) : String(displayFromCm(cm, unit))
+}
+
+/**
+ * Create (no garden) or edit (garden given) form. Dimensions are entered in the
+ * selected unit and converted to centimeters for the API; switching units
+ * converts the current values so the physical size is preserved. A 409 rebases
+ * the form onto the server's fresh row.
+ */
+export function GardenFormModal({ garden, onClose }: { garden?: Garden; onClose: () => void }) {
+  const isEdit = !!garden
+  const create = useCreateGarden()
+  const update = useUpdateGarden(garden?.id ?? 0)
+  const pending = create.isPending || update.isPending
+
+  const [name, setName] = useState(garden?.name ?? '')
+  const [unit, setUnit] = useState<UnitPref>(garden?.unitPref ?? 'metric')
+  const [width, setWidth] = useState(() => dimString(garden?.widthCm, garden?.unitPref ?? 'metric'))
+  const [height, setHeight] = useState(() => dimString(garden?.heightCm, garden?.unitPref ?? 'metric'))
+  const [notes, setNotes] = useState(garden?.notes ?? '')
+  const [version, setVersion] = useState(garden?.version ?? 0)
+  const [conflict, setConflict] = useState<string | null>(null)
+  const [formError, setFormError] = useState<string | null>(null)
+
+  function changeUnit(next: UnitPref) {
+    const convert = (s: string) => {
+      const v = parseFloat(s)
+      return Number.isFinite(v) ? String(displayFromCm(cmFromDisplay(v, unit), next)) : s
+    }
+    setWidth(convert(width))
+    setHeight(convert(height))
+    setUnit(next)
+  }
+
+  async function onSubmit(e: FormEvent) {
+    e.preventDefault()
+    setFormError(null)
+    setConflict(null)
+
+    const w = parseFloat(width)
+    const h = parseFloat(height)
+    if (!name.trim() || !Number.isFinite(w) || w <= 0 || !Number.isFinite(h) || h <= 0) {
+      setFormError('Enter a name and a positive width and height.')
+      return
+    }
+
+    const input = {
+      name: name.trim(),
+      widthCm: cmFromDisplay(w, unit),
+      heightCm: cmFromDisplay(h, unit),
+      unitPref: unit,
+      notes: notes.trim(),
+    }
+
+    try {
+      if (isEdit) {
+        await update.mutateAsync({ ...input, version })
+      } else {
+        await create.mutateAsync(input)
+      }
+      onClose()
+    } catch (err) {
+      const current = conflictGarden(err)
+      if (current) {
+        // Someone else changed this garden: rebase the form onto the fresh row so
+        // a re-save applies against the current version.
+        setVersion(current.version)
+        setName(current.name)
+        setUnit(current.unitPref)
+        setWidth(String(displayFromCm(current.widthCm, current.unitPref)))
+        setHeight(String(displayFromCm(current.heightCm, current.unitPref)))
+        setNotes(current.notes)
+        setConflict('This garden changed elsewhere. The latest values are shown — review and save again.')
+        return
+      }
+      setFormError(errorMessage(err, isEdit ? 'Could not save changes.' : 'Could not create the garden.'))
+    }
+  }
+
+  const unitLabel = dimensionUnitLabel(unit)
+
+  return (
+    <Modal title={isEdit ? 'Edit garden' : 'New garden'} onClose={onClose}>
+      <form onSubmit={onSubmit} className="flex flex-col gap-3">
+        {conflict && <Alert tone="info">{conflict}</Alert>}
+
+        <TextField label="Name" name="name" required value={name} onChange={(e) => setName(e.target.value)} />
+
+        <Select
+          label="Units"
+          name="unitPref"
+          value={unit}
+          onChange={(e) => changeUnit(e.target.value as UnitPref)}
+          options={unitOptions}
+        />
+
+        <div className="grid grid-cols-2 gap-3">
+          <TextField
+            label={`Width (${unitLabel})`}
+            name="width"
+            type="number"
+            inputMode="decimal"
+            step="any"
+            min="0"
+            required
+            value={width}
+            onChange={(e) => setWidth(e.target.value)}
+          />
+          <TextField
+            label={`Height (${unitLabel})`}
+            name="height"
+            type="number"
+            inputMode="decimal"
+            step="any"
+            min="0"
+            required
+            value={height}
+            onChange={(e) => setHeight(e.target.value)}
+          />
+        </div>
+
+        <TextArea label="Notes" name="notes" rows={2} value={notes} onChange={(e) => setNotes(e.target.value)} />
+
+        {formError && <Alert>{formError}</Alert>}
+
+        <div className="mt-1 flex justify-end gap-2">
+          <Button type="button" variant="ghost" onClick={onClose} disabled={pending}>
+            Cancel
+          </Button>
+          <Button type="submit" disabled={pending}>
+            {pending ? 'Saving…' : isEdit ? 'Save' : 'Create garden'}
+          </Button>
+        </div>
+      </form>
+    </Modal>
+  )
+}