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/desktop/constants/connection.ts

@@ -2,22 +2,14 @@
  * Connection-related constants for desktop app
  */
 
-// SaaS server URL from environment variable
-// The SaaS authentication server (Supabase)
-export const STIRLING_SAAS_URL: string = import.meta.env.VITE_SAAS_SERVER_URL || '';
+// SaaS authentication server URL
+export const STIRLING_SAAS_URL: string = import.meta.env.VITE_SAAS_SERVER_URL;
 
-// SaaS backend API URL from environment variable
-// The Stirling SaaS backend API server (for team endpoints, etc.)
-export const STIRLING_SAAS_BACKEND_API_URL: string = import.meta.env.VITE_SAAS_BACKEND_API_URL || '';
+// Stirling SaaS backend API server (for team endpoints, etc.)
+export const STIRLING_SAAS_BACKEND_API_URL: string = import.meta.env.VITE_SAAS_BACKEND_API_URL;
 
-// Supabase publishable key from environment variable
-// Used for SaaS authentication
-export const SUPABASE_KEY: string = import.meta.env.VITE_SUPABASE_PUBLISHABLE_DEFAULT_KEY || 'sb_publishable_UHz2SVRF5mvdrPHWkRteyA_yNlZTkYb';
+// Supabase publishable key — used for SaaS authentication
+export const SUPABASE_KEY: string = import.meta.env.VITE_SUPABASE_PUBLISHABLE_DEFAULT_KEY;
 
 // Desktop deep link callback for Supabase email confirmations
 export const DESKTOP_DEEP_LINK_CALLBACK = 'stirlingpdf://auth/callback';
-
-// Validation warnings
-if (!STIRLING_SAAS_BACKEND_API_URL) {
-  console.warn('[Desktop Connection] VITE_SAAS_BACKEND_API_URL not configured - SaaS backend APIs (teams, etc.) will not work');
-}

frontend/src/desktop/hooks/useEndpointConfig.ts

@@ -401,8 +401,7 @@ export function useMultipleEndpointsEnabled(endpoints: string[]): {
 // Default backend URL from environment variables
 const DEFAULT_BACKEND_URL =
   import.meta.env.VITE_DESKTOP_BACKEND_URL
-  || import.meta.env.VITE_API_BASE_URL
-  || '';
+  || import.meta.env.VITE_API_BASE_URL;
 
 /**
  * Desktop override exposing the backend URL based on connection mode.

frontend/src/desktop/services/apiClientConfig.ts

@@ -19,7 +19,7 @@ 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;
   }
 
   // In Tauri mode, return empty string as placeholder