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/README.md

@@ -1,6 +1,11 @@
-# Getting Started with Create React App
+# Frontend
+## Environment Variables
 
-This project was bootstrapped with [Create React App](https://github.com/facebook/create-react-app).
+The frontend requires environment variables to be set before running. `npm run dev` will create a `.env` file for you automatically on first run using the defaults from `config/.env.example` - for most development work this is all you need.
+
+If you need to configure specific services (Google Drive, Supabase, Stripe, PostHog), edit your local `.env` file. The values in `config/.env.example` show what each variable does and provides sensible defaults where applicable.
+
+For desktop (Tauri) development, `npm run tauri-dev` will additionally create a `.env.desktop` file from `config/.env.desktop.example`.
 
 ## Docker Setup
 
@@ -120,6 +125,11 @@ npm run tauri-dev
 
 This will run the gradle runboot command and the tauri dev command concurrently, starting the app once both are stable.
 
+> [!NOTE]
+>
+> Desktop builds require additional environment variables. See [Environment Variables](#environment-variables)
+> above - `npm run tauri-dev` will set these up automatically from `config/.env.desktop.example` on first run.
+
 ### Build
 To build a deployment of the Tauri app. Use this command in the `frontend` folder:
 
@@ -128,3 +138,8 @@ npm run tauri-build
 ```
 
 This will bundle the backend and frontend into one executable for each target. Targets can be set within the `tauri.conf.json` file.
+
+> [!NOTE]
+>
+> Desktop builds require additional environment variables. See [Environment Variables](#environment-variables)
+> above - `npm run tauri-build` will set these up automatically from `config/.env.desktop.example` on first run.