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.

web/vite.config.ts

@@ -1,7 +1,8 @@
+/// <reference types="vitest/config" />
 import { svelte } from '@sveltejs/vite-plugin-svelte'
 import tailwindcss from '@tailwindcss/vite'
-import { defineConfig } from 'vite'
 import { readFileSync, existsSync } from 'fs'
+import { defineConfig } from 'vite'
 
 // In Docker, VERSION is copied into the build WORKDIR (/build/web/VERSION).
 // In local dev, the cwd is web/ and VERSION is two dirs up (../../VERSION
@@ -50,5 +51,10 @@ export default defineConfig({
       // here so dev and prod agree on nomos's actual route paths.
       '/agent': authProxy('http://localhost:8092', (path) => path.replace(/^\/agent/, ''))
     }
+  },
+  test: {
+    environment: 'jsdom',
+    globals: true,
+    include: ['src/**/*.{test,spec}.{ts,js}']
   }
 })