ci(deploy): add gitea LXC deploy + validate workflows

Mirrors the buildfor_life_budget workflow pair: Gitea runs both
deploy and validate, GitHub mirrors validate only. Differences from
the sibling: pnpm + fnm instead of npm, Node pinned via .node-version,
and the repo is cloned over public HTTPS so no separate deploy key
is needed for git.b4l.co.th. Document required Gitea secrets in
DEPLOYMENT.md.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/validate.yml

@@ -0,0 +1,31 @@
+name: Validate
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup pnpm
+        uses: pnpm/action-setup@v4
+        with:
+          version: 9.15.0
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version-file: '.node-version'
+          cache: pnpm
+
+      - name: Install dependencies
+        run: pnpm install --frozen-lockfile
+
+      - name: Run validation (svelte-check + build)
+        run: pnpm run validate