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/notes/__init__.py

@@ -23,7 +23,7 @@ from sqlalchemy import func, literal_column, select
 
 from ..acl import visible_to_user
 from ..auth import login_required
-from ..colors import NOTE_COLORS, normalize_color
+from ..colors import normalize_color
 from ..common import coerce_bool, iso, parse_dt
 from ..config import Config
 from ..db import session_scope
@@ -108,7 +108,6 @@ async def list_notes():
     # Combinable facet filters (all optional, AND-ed together) — the rich-search /
     # saved-filter lens. Multiple ?label= narrow to notes carrying ALL of them.
     label_params = request.args.getlist("label")
-    color = request.args.get("color")
     has_reminder = coerce_bool(request.args.get("has_reminder"))
     has_attachment = coerce_bool(request.args.get("has_attachment"))
     query_text = (request.args.get("q") or "").strip()
@@ -128,10 +127,6 @@ async def list_notes():
             if lid is None:
                 return json_error("invalid label", 400)
             stmt = stmt.where(Note.id.in_(select(NoteLabel.note_id).where(NoteLabel.label_id == lid)))
-        if color is not None:
-            if color not in NOTE_COLORS:
-                return json_error("invalid color", 400)
-            stmt = stmt.where(Note.color == color)
         if has_reminder:
             stmt = stmt.where(Note.remind_at.is_not(None))
         if has_attachment:
@@ -259,7 +254,6 @@ async def export_notes():
                         "id": str(n.id),
                         "display_title": n.display_title,
                         "body": n.body,
-                        "color": n.color,
                         "pinned": n.pinned,
                         "archived": n.archived,
                         "remind_at": n.remind_at.isoformat() if n.remind_at else None,
@@ -412,7 +406,6 @@ async def create_note():
             owner_id=g.user_id,
             display_title=derive_display_title(body),
             body=body,
-            color=normalize_color(data.get("color")),
             position=int(max_pos) + 1,
         )
         db.add(note)
@@ -453,8 +446,6 @@ async def update_note(note_id: str):
         old_body = note.body
         if "body" in data and isinstance(data["body"], str):
             note.body = data["body"]
-        if "color" in data:
-            note.color = normalize_color(data["color"])
         if "pinned" in data:
             note.pinned = bool(data["pinned"])
         if "archived" in data: