Load color tag presets from an editable presets.json file

The presets move from app.js to presets.json, built into the binary and
served by /api/presets. PRESETS_FILE (default /data/presets.json) replaces
the list when present; it is read again each time Settings opens and the
built-in list is used if it is invalid. docs/presets.md (EN/FR) explains
each preset and the file format.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -4,7 +4,7 @@
 FROM golang:1.27.1-alpine3.24 AS build
 WORKDIR /src
 COPY go.mod ./
-COPY *.go ./
+COPY *.go presets.json ./
 COPY web ./web
 RUN CGO_ENABLED=0 go build -trimpath -ldflags="-s -w" -o /out/logstream .