fix(update-checker): improve error handling robustness

[Copilot]

Two error-handling deficiencies in the update checker's catch block that could hide useful diagnostic information or produce incorrect error categorization.

Changes

• Non-Error rejection handling: Replace hardcoded 'Failed to check for updates.' fallback with String(caught), preserving actual error details when the Tauri updater plugin rejects with a string or non-Error value — consistent with the String(error) pattern used elsewhere (e.g. App.tsx:566)

• Proxy validation error matching: Narrow lower.includes('invalid') to lower.includes('invalid update proxy url') — the broad check incorrectly routes unrelated errors (any message containing "invalid") to the "Update verification failed" bucket

// Before
const raw = caught instanceof Error ? caught.message : 'Failed to check for updates.';
// ...
lower.includes('invalid')

// After
const raw = caught instanceof Error ? caught.message : String(caught);
// ...
lower.includes('invalid update proxy url')