Fix Windows path resolution, add env variable overrides

[zcaceres]

Summary

• Replace new URL(import.meta.url).pathname with fileURLToPath(import.meta.url) to fix C:\C:\... double-drive-letter bug on Windows
• Add GOOGLE_OAUTH_KEYS_PATH env variable to override the OAuth keys file location
• Add GOOGLE_TASKS_CREDENTIALS_PATH env variable to override the saved credentials location
• Closes Windows: OAuth keyfilePath resolves incorrectly to C:\C:\... #7, Wrong command for Windows #10, Windows Path Resolution Bug for gcp-oauth.keys.json in gtasks-mcp #11, Feature Request: Make an env variable for the location of gcp-oath.keys.json #18

Test plan

• On Windows: bun run src/index.ts auth should resolve paths correctly (no C:\C:\...)
• With env vars: GOOGLE_OAUTH_KEYS_PATH=/custom/path.json bun run src/index.ts auth should use the custom path
• Without env vars: default relative paths still work as before