chore: delete the Flutter client — #2710

Superseded by the M8 native Android rewrite. Last touched 2026-05-31,
no workflow has built it since flutter.yml was removed, and rule #22
says a replaced path goes rather than lingering as something a reader
has to work out the status of. 245 files, ~24.6k lines.

Config references go with it: the .gitignore block (and its now-empty
"# Flutter" header), the .dockerignore entry, and renovate's ignorePaths
entry, which was suppressing dependency scanning for a directory that
no longer exists.

ci-requirements.md said ci-flutter "will retire once that directory
goes". It has gone, so the doc now says so -- CI-Runner can drop the
image, and nothing in this repo needs a Flutter toolchain.

One thing is kept rather than deleted: shared/fabledsword.tokens.json.
It lived under flutter_client/shared/ but was never Flutter's property
-- it is the canonical statement of the palette, the only place the
dark, light and flat cohorts are written down together, and
FabledSwordTokens.kt names it as its source of truth. Losing it would
have been collateral damage, so it moves to the repo root with a README
saying what it is and that neither client generates from it. That
comment in FabledSwordTokens.kt is repointed here.

What deliberately does NOT change: `runs-on: flutter-ci` in android.yml
and release.yml. That is a runner LABEL, not a path -- the Android jobs
schedule on it while pulling ci-android:36, per the label/image split
ci-requirements.md documents. Removing it would break scheduling for a
cosmetic win, so the doc now spells that out beside the retirement note.

Left for #2710: 64 files whose comments still name flutter_client/
paths. Sweeping them here would have buried the deletion, and each
needs a judgement -- keep the substance and drop the dead path, delete
pure "ported from" bookkeeping, or leave design rationale that happens
to mention the Flutter build.

.dockerignore

@@ -9,7 +9,6 @@ web/build
 # Flutter mobile client — built separately on developer machines / Flutter CI.
 # Including it in the Go build context wastes ~70 files and invalidates the
 # `COPY . .` layer cache on every Flutter-only change.
-flutter_client/
 
 # Docs and IDE noise
 docs/