feat(web): add eslint + prettier + vitest toolchain + web CI job (R8)

Added to web/package.json devDeps: eslint (9, flat config) +
eslint-plugin-svelte + typescript-eslint + globals; prettier +
prettier-plugin-svelte; vitest (jsdom env) + jsdom. New scripts: lint,
lint:fix, format, format:check, test, test:watch.

Configs:
- web/eslint.config.js — flat config, TS + Svelte, browser/node globals,
  no-explicit-any as warn, unused-vars as error (ignores _-prefixed).
- web/.prettierrc.json — single-quote, 100 width, svelte parser override.
- web/.prettierignore — dist/node_modules/build/lockfiles.
- web/vite.config.ts — vitest test block via reference directive, jsdom env,
  globals enabled.

Sample test: web/src/lib/utils.test.ts (6 tests covering relativeTime,
truncateMiddle, debounce — all passing).

CI: new web job in .gitea/workflows/ci.yml (npm ci, check [advisory],
lint [advisory], format:check [advisory], test [gate], build [gate]).
Advisory steps use continue-on-error until the baseline is clean —
matching the existing golangci-lint advisory pattern.

Known baseline surfaced by the new toolchain (pre-existing, not caused
by R8): svelte-check 154 errors (133-file config cascade), eslint 126
errors + 12 warnings (unused vars, @html XSS, unused CSS), prettier 175
unformatted files. Fixing these is a follow-up cleanup.

VERSION 0.7.9 -> 0.7.10. Plan R8 marked done; C.1 updated.

.gitea/workflows/ci.yml

@@ -70,3 +70,30 @@ jobs:
       - uses: actions/checkout@v4
       - name: docker build (verify image builds; no push)
         run: docker build -f compose/oikos/Dockerfile -t oikos:ci .
+
+  web:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: web
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '22'
+          cache: npm
+          cache-dependency-path: web/package-lock.json
+      - run: npm ci
+      - name: svelte-check (advisory — baseline not yet clean)
+        run: npm run check
+        continue-on-error: true
+      - name: eslint (advisory — baseline not yet clean)
+        run: npm run lint
+        continue-on-error: true
+      - name: prettier format check (advisory — baseline not yet clean)
+        run: npm run format:check
+        continue-on-error: true
+      - name: test
+        run: npm run test
+      - name: build
+        run: npm run build