Support configurable self-hosted API URLs

[DevOlabode]

Current behavior

src/shared/config.js and extension/options/options.html already have a developer-facing "Environment" + "API base URL" setting, but it's more limited than it looks:

• DEFAULT_API_URLS only knows about development (http://localhost:3000) and production (the hosted Render URL) - see src/shared/config.js:1-4.
• getApiBaseUrl() (src/shared/config.js:21-46) actively resets a non-localhost, non-legacy URL back to the hosted default whenever environment is production (config.js:38-42). In practice, a contributor typing a self-hosted URL into "Production" mode gets silently overridden.
• manifest.json hardcodes host_permissions (extension/manifest.json:34-37) and the CSP connect-src (extension/manifest.json:39) to just the hosted URL and localhost:3000. Even if the config layer allowed a custom URL, the browser would block the request - Manifest V3 host permissions and CSP connect-src are fixed at install/build time and can't be changed by a runtime setting.

This is a discussion issue before implementation

Manifest V3's optional_host_permissions lets an extension request additional host access at runtime via chrome.permissions.request(), prompting the user - that's the realistic path here, not editing host_permissions directly per install.

Please define, before implementing:

• Whether self-hosting is opt-in via optional_host_permissions (user grants access to their own host explicitly) rather than broadening the default host_permissions
• How the options page collects and validates a custom host (scheme required, no wildcards beyond what's needed)
• What happens to the CSP connect-src - it may need to become more permissive in a scoped way, or the request needs to go through a background-script relay
• How this interacts with the existing environment field, since today "production" actively fights a custom URL

Relevant files

• src/shared/config.js
• extension/options/options.html, extension/options/options.js
• extension/manifest.json (permissions, host_permissions, content_security_policy)
• docs/architecture.md ("no content scripts... Permissions are storage plus host access")

Non-goals

• Do not widen host_permissions to <all_urls> or a broad wildcard - that changes the extension's review/trust profile for every user, including the default hosted setup.