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/eslint.config.js

@@ -0,0 +1,39 @@
+import js from '@eslint/js'
+import ts from 'typescript-eslint'
+import svelte from 'eslint-plugin-svelte'
+import globals from 'globals'
+
+export default ts.config(
+  js.configs.recommended,
+  ...ts.configs.recommended,
+  ...svelte.configs['flat/recommended'],
+  {
+    files: ['**/*.{ts,js,svelte}'],
+    languageOptions: {
+      globals: {
+        ...globals.browser,
+        ...globals.node
+      }
+    }
+  },
+  {
+    files: ['**/*.svelte'],
+    languageOptions: {
+      parserOptions: {
+        parser: ts.parser
+      }
+    }
+  },
+  {
+    ignores: ['dist/', 'node_modules/', 'build/', '*.config.{ts,js}']
+  },
+  {
+    rules: {
+      '@typescript-eslint/no-explicit-any': 'warn',
+      '@typescript-eslint/no-unused-vars': [
+        'error',
+        { argsIgnorePattern: '^_', varsIgnorePattern: '^_' }
+      ]
+    }
+  }
+)