cards: one neutral surface, and the generated fill deleted with it

A card's fill stops being a function of the note. One neutral per theme on
all three surfaces — white in light, neutral-900 in dark, which is what the
palette's `default` always was and what both editors already used, so this
is a collapse onto a surface everything already had rather than a colour
anybody has to like.

Measured, against "not the same color as their background but close to it":
card vs board 1.04 light / 1.10 dark, edge vs card 1.98 / 1.73, body text
17.93 / 17.17, muted 10.37 / 14.23. The fill is deliberately the weakest
number on the card — the edge and the shadow separate it from the board, so
a fill that separated on its own would make it a panel.

Deleted, since the card was their only consumer: `derivedFill` / `hslHex`
and the level tables in colors.ts, `derivedFillArgb` / `hslToArgb` in
DerivedTint.kt, `NOTE_CARD_CLASSES_STRONG`, `chosenNoteColor`,
`noteCardClasses`, `noteTintVars`, the `.note-tint` rule in style.css,
`chosenBackground` / `tintable` / `noteTintFor` / `noteCardColor` /
`noteIsStrong` / `firstLabelColor` in NoteTint.kt, and
`resolvedNoteColor` / `noteColorIsChosen`.

`tintHash` and `derivedTint` STAY, against the plan: a label with no colour
of its own still derives one from its name, and that path was never the one
that failed. The mirrored pair and its fixture survive intact.

The editor follows the card, and its Done button takes the brand — the
board's compose FAB is the app's existing statement of "affirmative action
here", where Material's default secondaryContainer is a baseline colour this
theme never sets.

The colour picker is left in place, doing nothing, for exactly one step:
removing it here would leave `note.color` written by nothing and read by
nothing, which is a worse intermediate than a control that visibly does
nothing. #3041 takes the field and the picker together.

Step 1 of M315. #3148

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/src/style.css

@@ -47,26 +47,6 @@ body {
   }
 }
 
-/* An untagged note's fill is GENERATED from its id, not chosen from the palette —
- * see `derivedFill` in notes/colors.ts for why nine keys was never going to be
- * enough. That means it cannot be a Tailwind class, and it cannot be a plain inline
- * style either: light and dark are two different computed colours and an inline
- * style has no way to answer a media query. So the card publishes both as custom
- * properties and this rule picks between them.
- *
- * The fallbacks are the draft case. A note with no id yet has nothing to hash, so it
- * publishes no properties and lands on the plain card surface — one colour change at
- * save time, rather than every draft sharing a fill and then changing anyway. */
-.note-tint {
-  background-color: var(--tint-light, #ffffff);
-}
-
-@media (prefers-color-scheme: dark) {
-  .note-tint {
-    background-color: var(--tint-dark, #171717);
-  }
-}
-
 /* Motion is a feature, not a given. Anyone whose OS says "reduce motion" has told
  * us something about vestibular comfort or attention, and the answer is to arrive
  * instantly rather than to animate faster.