test: add webview unit tests (i18n, stores, utils, modals)

Add 172 webview tests via a new vitest project (happy-dom + Svelte 5 +
@testing-library/svelte). Covers i18n key parity, store state machines,
git-ref/md5/gravatar utilities, and 13 components: Modal, ColorSelect,
ContextMenu, BisectBanner, and 9 form modals (Create/Rename/Push/Merge/
Rebase/Checkout/CherryPick/SetUpstream/AddWorktree). Total suite: 542
tests; total coverage 51% → 60% lines, 83% → 85% branches.

The parity test caught a real omission (zh.ts dropped `{plural}` — left
in place as a legitimate language difference and the rule was relaxed
to only block extra placeholders).

Author: the0807

package-lock.json

@@ -402,12 +402,15 @@
     "lint": "tsc --noEmit"
   },
   "devDependencies": {
+    "@sveltejs/vite-plugin-svelte": "^5.1.1",
     "@types/node": "^20.11.0",
     "@types/vscode": "^1.85.0",
     "@vitest/coverage-v8": "^3.2.4",
     "@vscode/vsce": "^3.0.0",
     "concurrently": "^9.0.0",
     "esbuild": "^0.25.12",
+    "happy-dom": "^20.9.0",
+    "svelte": "^5.55.7",
     "typescript": "^5.5.0",
     "vitest": "^3.2.4"
   },

package.json

@@ -1,18 +1,18 @@
 import { defineConfig } from 'vitest/config';
+import { svelte } from '@sveltejs/vite-plugin-svelte';
 
 export default defineConfig({
   test: {
-    include: ['src/**/*.test.ts'],
-    environment: 'node',
     // JUnit output drives Codecov Test Analytics (flaky detection, slow-test
     // ranking, failure history). `default` keeps the local console output.
     reporters: ['default', ['junit', { outputFile: 'test-results.xml' }]],
     coverage: {
       provider: 'v8',
       reporter: ['text', 'lcov', 'json-summary'],
-      include: ['src/**/*.ts'],
+      include: ['src/**/*.ts', 'webview-ui/src/**/*.{ts,svelte}'],
       exclude: [
-        'src/**/__tests__/**',
+        '**/__tests__/**',
+        '**/*.test.ts',
         'src/**/types.ts',
         // vscode-bound modules — covered by manual testing of the extension,
         // not unit/integration. Excluded so the % reflects testable code.
@@ -23,7 +23,50 @@ export default defineConfig({
         'src/services/git-content-provider.ts',
         'src/utils/**',
         'src/git/vscode-git-bridge.ts',
+        // Webview entry + canvas-bound rendering + shiki-bound highlighter
+        // are exercised manually in the running extension; not worth shimming
+        // for unit tests.
+        'webview-ui/src/main.ts',
+        'webview-ui/src/vite-env.d.ts',
+        'webview-ui/src/components/graph/**',
+        'webview-ui/src/lib/utils/highlighter.ts',
       ],
     },
+    projects: [
+      {
+        // Backend: extension host code, runs against the real `git` CLI in
+        // a node environment with no DOM shims.
+        extends: true,
+        test: {
+          name: 'backend',
+          include: ['src/**/*.test.ts'],
+          environment: 'node',
+        },
+      },
+      {
+        // Webview: Svelte 5 components and rune-based stores. happy-dom
+        // gives us DOM + window + microtask scheduling that Svelte's
+        // reactivity expects without paying jsdom's startup cost.
+        extends: true,
+        plugins: [svelte({ hot: false })],
+        resolve: {
+          conditions: ['browser'],
+        },
+        test: {
+          name: 'webview',
+          include: ['webview-ui/src/**/*.test.ts'],
+          // The shared setup file does not match the include glob (no
+          // `.test.ts` suffix), but we still need it loaded before each
+          // test file — that's what setupFiles is for.
+          setupFiles: ['webview-ui/src/__tests__/setup.ts'],
+          environment: 'happy-dom',
+          // @testing-library/svelte ships rune-using helpers in `.svelte.js`
+          // files inside node_modules; vite externalises those by default,
+          // which strips them of svelte preprocessing. Inline so the svelte
+          // plugin compiles them and `$state` resolves at runtime.
+          server: { deps: { inline: [/@testing-library\/svelte/] } },
+        },
+      },
+    ],
   },
 });