Smoke-sweep fixes: exact saves, local dates, safer remove, readable markers

- Garden and plant dialogs keep centimeters as the source of truth
  (LengthField in lib/units.ts): a no-change Save no longer rewrites
  900 cm as 899.922 or a 45 cm spacing as 44.958, bumping versions and
  writing bogus history entries on the way.
- The UI stamps every date with the browser's local day (lib/dates.ts).
  Journal notes already did; plop placement, fill and removal now do too,
  so a 9 pm placement isn't "planted tomorrow". The fill endpoint gained an
  optional plantedAt; API and agent callers still default to UTC today.
- Removing an object that holds plants asks first and says how many go
  with it. An empty one still goes straight away (one Undo restores it).
- The expanded plant card's action row wraps instead of clipping "Delete".
- Monogram lettering switches to a dark ink on pale marker colors (garlic,
  cabbage, marigold) instead of near-white on near-white.
- Copy-as-plan proposes the next free year and warns when the typed name
  already exists, so two gardens can't both read as "the 2027 plan".
- Plan cards show the base name with a "2027 plan" tag, so the year — the
  point of the name — survives truncation.
- A rejected model spec now says which model and why: a wrapped
  ErrInvalidInput's reason reaches the client as the 400's message, and the
  Settings field shows it inline instead of toasting "invalid input".

Also defuses a clock bomb in TestRemainingReturnsWhenAPlantingIsRemoved,
which only passed while the real date was before 2026-08-01.

Co-Authored-By: Claude Fable 5 <[email protected]>

web/src/editor/Canvas.tsx

@@ -10,6 +10,7 @@ import {
   type PointerEvent as ReactPointerEvent,
 } from 'react'
 import { clampScale, type Point } from '@/lib/geometry'
+import { monogramInk } from '@/lib/monogram'
 import { useCreateObject, useCreatePlanting, useUpdateObject, useUpdatePlanting } from '@/lib/objects'
 import type { Plant } from '@/lib/plants'
 import type { EditorPlanting } from '@/lib/plantings'
@@ -34,6 +35,10 @@ import {
 import { useEditorStore, type Viewport } from './store'
 import type { EditorGarden, EditorObject } from './types'
 
+// A plop whose plant is missing from the catalog (a shared garden's private
+// plant) still needs a color to be drawn in.
+const FALLBACK_PLANT_COLOR = '#97a97c'
+
 const WHEEL_SENSITIVITY = 0.0016
 const ANIM_MS = 520
 const REFIT_THRESHOLD_PX = 60
@@ -589,7 +594,7 @@ export const Canvas = forwardRef<
               >
                 {/* A fingertip-sized hit area so a tiny plop at low zoom is still grabbable. */}
                 <circle r={Math.max(p.radiusCm, 10 / s)} fill="transparent" />
-                <circle r={p.radiusCm} fill={plant?.color ?? '#97a97c'} stroke={isSel ? 'var(--color-paper)' : 'none'} strokeWidth={2.5 / s} />
+                <circle r={p.radiusCm} fill={plant?.color ?? FALLBACK_PLANT_COLOR} stroke={isSel ? 'var(--color-paper)' : 'none'} strokeWidth={2.5 / s} />
               </g>
             )
           })}
@@ -642,7 +647,7 @@ export const Canvas = forwardRef<
                     textAnchor="middle"
                     dominantBaseline="central"
                     fontSize={r * 1.05}
-                    fill="var(--color-paper)"
+                    fill={monogramInk(plant?.color ?? FALLBACK_PLANT_COLOR)}
                     style={{ fontFamily: 'var(--font-heading)' }}
                   >
                     {letters.get(p.plantId) ?? '?'}