fix: store instance settings in extension storage instead of localStorage

[Laecherlich]

Problem

With a self-hosted instance configured, the injected issue-page buttons (Jira/Linear/Plane) fail with a 401 and log the user out.

endpoint and clientId are kept in useStorage (vueuse), which is backed by localStorage. Content scripts share localStorage with the host page (e.g. *.atlassian.net), where these keys never exist — so all API calls from injected buttons silently fall back to the default cloud endpoint. The instance's Bearer token (correctly shared via browser.storage) is then sent to app.solidtime.io, which returns 401; the interceptor's token refresh also targets the wrong host, fails, and clears the session.

Fix

• Store instance_endpoint / instance_client_id in browser.storage.local, exactly like the tokens already are.
• Expose a settingsLoaded promise; the content script awaits it before any integration makes API calls.
• Sync changes across contexts via the existing storage.onChanged listener.
• Migrate values a previous version persisted in the popup's localStorage (extension pages only — a content script's localStorage belongs to the host page and must not be migrated from).

Adds no new vue-tsc errors (same error count as main).