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/lib/monogram.test.ts

@@ -1,5 +1,5 @@
 import { describe, expect, it } from 'vitest'
-import { monogramFor, monogramMap, speciesName } from './monogram'
+import { monogramFor, monogramInk, monogramMap, speciesName } from './monogram'
 
 describe('speciesName', () => {
   it('drops a variety suffix and a parenthetical', () => {
@@ -65,3 +65,22 @@ describe('monogramMap', () => {
     expect(monogramFor('🌱')).toBe('?')
   })
 })
+
+describe('monogramInk', () => {
+  it('keeps paper lettering on colors dark enough to carry it', () => {
+    expect(monogramInk('#c8553d')).toBe('var(--color-paper)') // tomato
+    expect(monogramInk('#7a8a5e')).toBe('var(--color-paper)') // sage
+    expect(monogramInk('#5f8f45')).toBe('var(--color-paper)')
+  })
+
+  it('switches to the dark marker ink on pale colors', () => {
+    expect(monogramInk('#d9d2c5')).toBe('var(--color-marker-ink)') // garlic
+    expect(monogramInk('#8bc98b')).toBe('var(--color-marker-ink)') // cabbage
+    expect(monogramInk('#fff')).toBe('var(--color-marker-ink)')
+  })
+
+  it('falls back to paper for anything it cannot read', () => {
+    expect(monogramInk('tomato')).toBe('var(--color-paper)')
+    expect(monogramInk('')).toBe('var(--color-paper)')
+  })
+})