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.ts

@@ -61,3 +61,34 @@ export function monogramFor(name: string): string {
   const ls = letters(name)
   return ls.length ? ls[0].toUpperCase() : '?'
 }
+
+// --- Lettering color --------------------------------------------------------
+// Letters are paper on the marker's color — which reads on tomato red and sage,
+// and vanishes on garlic's #d9d2c5. Pale markers take the dark marker ink
+// instead. Both inks are theme-stable (the marker's own color is), so the
+// choice depends only on the color, never on light/dark mode.
+
+const PAPER = 'var(--color-paper)'
+const INK = 'var(--color-marker-ink)'
+// Below this relative luminance, paper still clears ~2.5:1 against the marker;
+// above it, the dark ink does better. Sage (#7a8a5e, 0.23) keeps paper; cabbage
+// green (#8bc98b, 0.49), marigold orange and garlic flip to ink.
+const PAPER_MAX_LUMINANCE = 0.37
+
+/** WCAG relative luminance of a #rgb / #rrggbb color; null if unparseable. */
+function luminance(color: string): number | null {
+  const m = color.trim().match(/^#?([0-9a-f]{3}|[0-9a-f]{6})$/i)
+  if (!m) return null
+  const hex = m[1].length === 3 ? [...m[1]].map((c) => c + c).join('') : m[1]
+  const channel = (i: number) => {
+    const v = parseInt(hex.slice(i, i + 2), 16) / 255
+    return v <= 0.03928 ? v / 12.92 : ((v + 0.055) / 1.055) ** 2.4
+  }
+  return 0.2126 * channel(0) + 0.7152 * channel(2) + 0.0722 * channel(4)
+}
+
+/** The CSS color the monogram letters take on a marker of `color`. */
+export function monogramInk(color: string): string {
+  const l = luminance(color)
+  return l !== null && l > PAPER_MAX_LUMINANCE ? INK : PAPER
+}