fix(settings): make the settings contract enforceable and fix the appearance cache

The contract manifest landed as a document nothing checked. This makes it a
mechanism, and fixes the one defect in the change set that hurt users on merge
rather than at cutover.

Web appearance cache. useTheme cleared the cache for any account whose stamp
did not match and never repopulated it — the only writers were the four
user-action setters — so every upgrading user lost their warm start on every
load, not once, and x-large-text and high-contrast users lost theirs too. The
owner-stamp protocol is replaced with per-account key namespacing
(`silo-theme:7`): a foreign value is absent rather than present-and-distrusted,
so nothing has to be deleted, the first account keeps its warm start, and there
is no shared stamp for a second tab, a stale debounce timer, or an out-of-order
effect to race on. Widening ownership to profile scope, which this manifest
requires, is now a change to appearanceCacheOwner alone. Adds the API-to-cache
mirror useTheme was missing, cancels pending debounced writes across an account
change, and re-seeds provider state during render so no frame paints the
previous account's look.

Canonicalization. writeCanonical used json.Marshal, which HTML-escapes < > and
&, and canonicalNumber used Go's 'g' format — both diverge from RFC 8785, so
the first label containing an ampersand or bound below 1e-4 would have forked
the server's ETag from every conforming client. Output is now byte-identical to
ECMAScript String() across the edge cases, verified against node. The ETag also
covers the value schemas, which decide what the server accepts and previously
could change while the tag stood still. All four derived representations are
memoized; a conditional GET no longer costs a full parse and re-serialize.

Validation. strictUnmarshal's decoder.More() answered false for a stray ] or },
so `true]` validated as a boolean. Enum matching compared fmt.Sprintf tokens, so
the string "3" satisfied an integer member. Declared steps were never enforced.
The language pattern rejected tags both mobile platforms emit unprompted
(en_US, ca-ES-valencia, ar-EG-u-nu-latn) and never normalized case, so en-US and
en-us were two rows for one preference; NormalizeValue now canonicalizes on the
shared path.

Manifest. show_forced_subtitles defaulted false where the server column is NOT
NULL DEFAULT true, which would have turned forced subtitles off for every
profile that never touched it. preferred_quality declared 13 members where the
planner speaks 6 and collapses the rest to auto. metadata_language's allowlist
was bound to the very column it migrates from. subtitle-appearance pinned
fontFamily to three families while Apple stores any installed system font.
Registers five user-facing settings the clients already ship, and corrects three
notes that described Android behaviour that was not true.

Enforcement. The package had no non-test callers, so MustLoad never ran; it now
loads and logs at startup. The inventory test compared the manifest against a
hand-copied map and could not see the drift it named; it now iterates
settingsRegistry and checks defaults too — both verified to fail on injected
drift. Adds .github/workflows/ci.yml, the repo's first CI that runs go test,
go vet, gofmt, and the frontend suite. Known pre-existing failures are named
individually in the Makefile so everything else stays gated and the list can
only shrink.

Part of #135

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -0,0 +1,107 @@
+name: CI
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+  workflow_dispatch:
+
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: true
+
+env:
+  GOPROXY: https://proxy.golang.org,direct
+  GOPRIVATE: github.com/Silo-Server/*
+  GONOSUMDB: github.com/Silo-Server/*
+
+jobs:
+  go:
+    name: Go
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v5
+
+      - name: Set up Go
+        uses: actions/setup-go@v6
+        with:
+          go-version-file: go.mod
+          cache: true
+
+      # cmd/silo embeds the built frontend, so nothing under ./... compiles
+      # without web/dist. The Go jobs never serve it, so a placeholder is
+      # enough; the Docker workflow builds the real bundle.
+      - name: Stub the embedded frontend bundle
+        run: make embed-stub
+
+      - name: Build
+        run: go build ./...
+
+      - name: gofmt
+        run: |
+          unformatted="$(gofmt -l .)"
+          if [ -n "$unformatted" ]; then
+            echo "::error::gofmt is required on:"
+            echo "$unformatted"
+            exit 1
+          fi
+
+      - name: Vet
+        run: go vet ./...
+
+      # Runs the settings-contract gate among everything else: the embedded
+      # manifest must parse, satisfy its own schema, hold every structural
+      # invariant, and agree with the live settings registry on keys and
+      # defaults. Without this job those tests exist but never run.
+      - name: Test
+        run: make test-go
+
+  web:
+    name: Web
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: web
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v5
+
+      - name: Set up pnpm
+        uses: pnpm/action-setup@v4
+
+      - name: Set up Node
+        uses: actions/setup-node@v5
+        with:
+          node-version: 22
+          cache: pnpm
+          cache-dependency-path: web/pnpm-lock.yaml
+
+      - name: Install
+        run: pnpm install --frozen-lockfile
+
+      - name: Lint
+        run: pnpm run lint
+
+      - name: Format check
+        run: pnpm run format:check
+
+      - name: Typecheck and build
+        run: pnpm run build
+
+      # Includes the appearance-cache ownership tests, which are the regression
+      # guard for cross-account leaks in the localStorage warm start.
+      - name: Test
+        working-directory: .
+        run: make test-web
+
+  docs:
+    name: Docs hygiene
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v5
+
+      - name: Verify no local paths leaked into committed docs
+        run: make verify-local-paths