notes: color leaves the model, the wire and all three surfaces

Step 3 of M315, and the destructive half. Steps 1 and 2 stopped every read of
this field: a card is one neutral surface per theme, and the only coloured
thing on a board is a tag. What was left was a column written by a picker and
read by nothing.

Rule 22 — the old path comes out completely. No flag, no fallback, no
"override if set".

Server: the column, the `?color=` facet, the create/update/serialise paths,
the sync assignment, the front-matter line, and Keep's colour map. Alembic
0029 drops it and sweeps `"color"` out of stored saved-filter params — a view
that silently filtered on a field the app no longer has would return nothing
and never say why. That sweep is Python, not `params::jsonb - 'color'`,
because Postgres has no try-cast and one malformed blob would abort a
migration that is running over somebody's saved views.

`NOTE_COLORS` moves from `models/note.py` to `colors.py`. A palette defined on
the model that lost one is an invitation to put the column back; labels still
name a colour, so the vocabulary belongs where the normalizer already is.

Core: the field, the facet, the `NoteCreateInput`, and every read and write in
store/push/pull. Local schema v9 drops the column and does the same
saved-filter sweep, guarded on `json_valid` so a corrupt blob loses a key
rather than becoming NULL. The uniffi layer drops `NoteEdit::Color` and
`NoteDraft.color` with it.

Web: `ColorPicker.vue`, the per-card swatch popover and its stylesheet rule,
the FilterBar colour row, the facet in the query round-trip, and the colour
half of the editor's baseline-and-save. Android: the `ColorSheet`, the
`Picker.COLOR` case, the toolbar's swatch dot, `EditorAction.SetColor`.

## The protocol: v4, and the floor deliberately stays at 3

Checked against `compat.rs` and the push handler rather than trusting the
`#[serde(default)]` annotation, because the v2 precedent points the other way:
v2 dropped `kind` and `title` and DID raise both floors, on the rule that
dropping a field a client sends and expects back is breaking.

`color` fails the second half of that test. A v3 client reading a v4 note gets
`"default"` from its own serde default and draws the colour it derives
locally — the board it drew yesterday. A v3 client pushing `color` has the key
ignored, since `_assign_note_fields` reads its payload key by key and never
validates the shape. Neither direction errors and neither shows anything
wrong. `title` was the note's NAME; this is a field that no longer renders.

So `SYNC_PROTOCOL_VERSION` and `CLIENT_PROTOCOL_VERSION` go to 4, and both
floors stay at 3. `docs/sync.md` carries the reasoning and the per-version
history, and its push example is brought back in line — it still listed
`title`, `kind` and `items`, all gone before this.

Import stays tolerant: a pre-M315 export or a Keep takeout carrying `color:`
imports fine, the key simply read past. Old exports must still import.

#3041

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

src/thoughtsync/colors.py

@@ -1,16 +1,36 @@
 from __future__ import annotations
 
-from .models.note import NOTE_COLORS
-
-# Notes and labels share one colour palette (their sets were identical). NOTE_COLORS
-# is the canonical vocabulary (defined on the model); this module is the single home
-# for the "clamp to the palette" normalizer so notes.py, labels.py and sync.py stop
-# each carrying their own copy.
+# The colour palette, and the one place that clamps to it.
+#
+# It lived on `models/note.py` until M315, when a note stopped having a colour. A
+# palette defined on the model that lost one would be a standing invitation to put the
+# column back; here it reads as what it now is — a LABEL's vocabulary, shared with the
+# saved-filter and import paths that still name a colour.
+#
+# Keys, not tints. The actual colours live in each client (frontend/src/notes/colors.ts
+# and NoteTint.kt), so they can be retuned without a schema migration — which M315 spent
+# two steps doing.
+NOTE_COLORS = {
+    "default",
+    "red",
+    "orange",
+    "yellow",
+    "green",
+    "teal",
+    "blue",
+    "purple",
+    "pink",
+    "gray",
+}
 
 __all__ = ["NOTE_COLORS", "normalize_color"]
 
 
 def normalize_color(color: object) -> str:
-    """Return `color` if it's a known palette key, else the default. One definition
-    for both notes and labels."""
+    """Return `color` if it's a known palette key, else the default.
+
+    `default` is no longer something anybody can CHOOSE — nothing offers a colour
+    picker since M315 — but it is still where unrecognised input has to land, so this
+    fallback is unreachable by choice rather than dead.
+    """
     return color if color in NOTE_COLORS else "default"