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/package-lock.json

@@ -104,6 +104,7 @@
         "@typescript-eslint/parser": "^8.44.1",
         "@vitejs/plugin-react-swc": "^4.1.0",
         "@vitest/coverage-v8": "^3.2.4",
+        "dotenv": "^16.4.7",
         "dpdm": "^3.14.0",
         "eslint": "^10.0.2",
         "jsdom": "^27.0.0",
@@ -114,6 +115,7 @@
         "postcss-preset-mantine": "^1.18.0",
         "postcss-simple-vars": "^7.0.1",
         "puppeteer": "^24.25.0",
+        "tsx": "^4.19.4",
         "typescript": "^5.9.2",
         "typescript-eslint": "^8.44.1",
         "vite": "^7.1.7",
@@ -7518,6 +7520,19 @@
         "@types/trusted-types": "^2.0.7"
       }
     },
+    "node_modules/dotenv": {
+      "version": "16.6.1",
+      "resolved": "https://registry.npmjs.org/dotenv/-/dotenv-16.6.1.tgz",
+      "integrity": "sha512-uBq4egWHTcTt33a72vpSG0z3HnPuIl6NqYcTrKEg2azoEyl2hpW0zqlxysq2pK9HlDIHyHyakeYaYnSAwd8bow==",
+      "dev": true,
+      "license": "BSD-2-Clause",
+      "engines": {
+        "node": ">=12"
+      },
+      "funding": {
+        "url": "https://dotenvx.com"
+      }
+    },
     "node_modules/dpdm": {
       "version": "3.15.1",
       "resolved": "https://registry.npmjs.org/dpdm/-/dpdm-3.15.1.tgz",
@@ -8540,6 +8555,19 @@
         "url": "https://github.com/sponsors/sindresorhus"
       }
     },
+    "node_modules/get-tsconfig": {
+      "version": "4.13.6",
+      "resolved": "https://registry.npmjs.org/get-tsconfig/-/get-tsconfig-4.13.6.tgz",
+      "integrity": "sha512-shZT/QMiSHc/YBLxxOkMtgSid5HFoauqCE3/exfsEcwg1WkeqjG+V40yBbBrsD+jW2HDXcs28xOfcbm2jI8Ddw==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "resolve-pkg-maps": "^1.0.0"
+      },
+      "funding": {
+        "url": "https://github.com/privatenumber/get-tsconfig?sponsor=1"
+      }
+    },
     "node_modules/get-uri": {
       "version": "6.0.5",
       "resolved": "https://registry.npmjs.org/get-uri/-/get-uri-6.0.5.tgz",
@@ -12145,6 +12173,16 @@
         "node": ">=4"
       }
     },
+    "node_modules/resolve-pkg-maps": {
+      "version": "1.0.0",
+      "resolved": "https://registry.npmjs.org/resolve-pkg-maps/-/resolve-pkg-maps-1.0.0.tgz",
+      "integrity": "sha512-seS2Tj26TBVOC2NIc2rOe2y2ZO7efxITtLZcGSOnHHNOQ7CkiUBfw0Iw2ck6xkIhPwLhKNLS8BO+hEpngQlqzw==",
+      "dev": true,
+      "license": "MIT",
+      "funding": {
+        "url": "https://github.com/privatenumber/resolve-pkg-maps?sponsor=1"
+      }
+    },
     "node_modules/responselike": {
       "version": "4.0.2",
       "resolved": "https://registry.npmjs.org/responselike/-/responselike-4.0.2.tgz",
@@ -13384,6 +13422,41 @@
       "integrity": "sha512-oJFu94HQb+KVduSUQL7wnpmqnfmLsOA/nAh6b6EH0wCEoK0/mPeXU6c3wKDV83MkOuHPRHtSXKKU99IBazS/2w==",
       "license": "0BSD"
     },
+    "node_modules/tsx": {
+      "version": "4.21.0",
+      "resolved": "https://registry.npmjs.org/tsx/-/tsx-4.21.0.tgz",
+      "integrity": "sha512-5C1sg4USs1lfG0GFb2RLXsdpXqBSEhAaA/0kPL01wxzpMqLILNxIxIOKiILz+cdg/pLnOUxFYOR5yhHU666wbw==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "esbuild": "~0.27.0",
+        "get-tsconfig": "^4.7.5"
+      },
+      "bin": {
+        "tsx": "dist/cli.mjs"
+      },
+      "engines": {
+        "node": ">=18.0.0"
+      },
+      "optionalDependencies": {
+        "fsevents": "~2.3.3"
+      }
+    },
+    "node_modules/tsx/node_modules/fsevents": {
+      "version": "2.3.3",
+      "resolved": "https://registry.npmjs.org/fsevents/-/fsevents-2.3.3.tgz",
+      "integrity": "sha512-5xoDfX+fL7faATnagmWPpbFtwh/R77WmMMqqHGS65C3vvB0YHrgF+B1YmZ3441tMj5n63k0212XNoJwzlhffQw==",
+      "dev": true,
+      "hasInstallScript": true,
+      "license": "MIT",
+      "optional": true,
+      "os": [
+        "darwin"
+      ],
+      "engines": {
+        "node": "^8.16.0 || ^10.6.0 || >=11.0.0"
+      }
+    },
     "node_modules/type-check": {
       "version": "0.4.0",
       "resolved": "https://registry.npmjs.org/type-check/-/type-check-0.4.0.tgz",