Add system for managing env vars (#5902)

# Description of Changes
Previously, `VITE_*` environment variables were scattered across the
codebase with hardcoded fallback values inline (e.g.
`import.meta.env.VITE_STRIPE_KEY || 'pk_live_...'`). This made it
unclear which variables
were required, what they were for, and caused real keys to be silently
used in builds where they hadn't been explicitly configured.

## What's changed

I've added `frontend/.env.example` and `frontend/.env.desktop.example`,
which declare every `VITE_*` variable the app uses, with comments
explaining each one and sensible defaults where applicable. These
are the source of truth for what's required.

I've added a setup script which runs before `npm run dev`, `build`,
`tauri-dev`, and all `tauri-build*` commands. It:
- Creates your local `.env` / `.env.desktop` from the example files on
first run, so you don't need to do anything manually
- Errors if you're missing keys that the example defines (e.g. after
pulling changes that added a new variable). These can either be
manually-set env vars, or in your `.env` file (env vars take precedence
over `.env` file vars when running)
- Warns if you have `VITE_*` variables set in your environment that
aren't listed in any example file

I've removed all `|| 'hardcoded-value'` defaults from source files
because they are not necessary in this system, as all variables must be
explicitly set (they can be set to `VITE_ENV_VAR=`, just as long as the
variable actually exists). I think this system will make it really
obvious exactly what you need to set and what's actually running in the
code.

I've added a test that checks that every `import.meta.env.VITE_*`
reference found in source is present in at least one example file, so
new variables can't be added without being documented.

## For contributors

New contributors shouldn't need to do anything - `npm run dev` will
create your `.env` automatically.

If you already have a `.env` file in the `frontend/` folder, you may
well need to update it to make the system happy. Here's an example
output from running `npm run dev` with an old `.env` file:

```
$ npm run dev

> [email protected] dev
> npm run prep && vite


> [email protected] prep
> tsx scripts/setup-env.ts && npm run generate-icons

setup-env: see frontend/README.md#environment-variables for documentation
setup-env: .env is missing keys from config/.env.example:
  VITE_GOOGLE_DRIVE_CLIENT_ID
  VITE_GOOGLE_DRIVE_API_KEY
  VITE_GOOGLE_DRIVE_APP_ID
  VITE_PUBLIC_POSTHOG_KEY
  VITE_PUBLIC_POSTHOG_HOST
  Add them manually or delete your local file to re-copy from the example.
setup-env: the following VITE_ vars are set but not listed in any example file:
  VITE_DEV_BYPASS_AUTH
  Add them to config/.env.example or config/.env.desktop.example if they are required.
```

If you add a new `VITE_*` variable to the codebase, add it to the
appropriate `frontend/config/.env.example` file or the test will fail.

frontend/src/core/env.test.ts

@@ -0,0 +1,58 @@
+import { readFileSync, readdirSync, statSync } from 'fs';
+import { join, extname } from 'path';
+import { fileURLToPath } from 'url';
+import { describe, it, expect } from 'vitest';
+
+// frontend/ root — this file lives at src/core/env.test.ts
+const frontendRoot = join(fileURLToPath(import.meta.url), '../../..');
+
+function parseEnvKeys(content: string): Set<string> {
+  const keys = new Set<string>();
+  for (const line of content.split('\n')) {
+    const trimmed = line.trim();
+    if (trimmed && !trimmed.startsWith('#')) {
+      const match = trimmed.match(/^([A-Z_][A-Z0-9_]*)=/);
+      if (match) keys.add(match[1]);
+    }
+  }
+  return keys;
+}
+
+function collectSourceFiles(dir: string): string[] {
+  const files: string[] = [];
+  for (const entry of readdirSync(dir)) {
+    const fullPath = join(dir, entry);
+    const stat = statSync(fullPath);
+    if (stat.isDirectory() && entry !== 'node_modules' && entry !== 'assets') {
+      files.push(...collectSourceFiles(fullPath));
+    } else if (stat.isFile() && (extname(entry) === '.ts' || extname(entry) === '.tsx') && !entry.endsWith('.d.ts')) {
+      files.push(fullPath);
+    }
+  }
+  return files;
+}
+
+function findViteEnvVars(srcDir: string): Set<string> {
+  const vars = new Set<string>();
+  for (const file of collectSourceFiles(srcDir)) {
+    const content = readFileSync(file, 'utf-8');
+    for (const match of content.matchAll(/import\.meta\.env\.(VITE_\w+)/g)) {
+      vars.add(match[1]);
+    }
+  }
+  return vars;
+}
+
+describe('env vars', () => {
+  it('every VITE_ var used in source is present in an example env file', () => {
+    const baseEnv = readFileSync(join(frontendRoot, 'config/.env.example'), 'utf-8');
+    const desktopEnv = readFileSync(join(frontendRoot, 'config/.env.desktop.example'), 'utf-8');
+    const saasEnv = readFileSync(join(frontendRoot, 'config/.env.saas.example'), 'utf-8');
+
+    const exampleKeys = new Set([...parseEnvKeys(baseEnv), ...parseEnvKeys(desktopEnv), ...parseEnvKeys(saasEnv)]);
+    const sourceVars = findViteEnvVars(join(frontendRoot, 'src'));
+
+    const missing = [...sourceVars].filter(v => !exampleKeys.has(v));
+    expect(missing, `Missing from 'frontend/config/.env.example' files: ${missing.join(', ')}`).toHaveLength(0);
+  });
+});

frontend/src/core/services/apiClientConfig.ts

@@ -15,5 +15,5 @@ export function getApiBaseUrl(): string {
     return (window as any).STIRLING_PDF_API_BASE_URL;
   }
 
-  return import.meta.env.VITE_API_BASE_URL || '/';
+  return import.meta.env.VITE_API_BASE_URL;
 }

frontend/src/core/services/supabaseClient.ts

@@ -1,7 +1,7 @@
 import { createClient, SupabaseClient } from '@supabase/supabase-js';
 
-const supabaseUrl = import.meta.env.VITE_SUPABASE_URL || 'https://rficokptxxxxtyzcvgmx.supabase.co';
-const supabaseAnonKey = import.meta.env.VITE_SUPABASE_PUBLISHABLE_DEFAULT_KEY || 'sb_publishable_UHz2SVRF5mvdrPHWkRteyA_yNlZTkYb';
+const supabaseUrl = import.meta.env.VITE_SUPABASE_URL;
+const supabaseAnonKey = import.meta.env.VITE_SUPABASE_PUBLISHABLE_DEFAULT_KEY;
 
 // Check if Supabase is configured
 export const isSupabaseConfigured = !!(supabaseUrl && supabaseAnonKey);