Exit when the HTTP server fails to start

[Nameless-Monster-Nerd]

Fixes #950.

Server startup errors from the watched-config path now flow back to serveApp through a buffered error channel. This makes Main return a non-zero exit code for failures such as an occupied or unavailable bind address instead of logging the error and waiting forever. Initial config and application construction failures now use the same error path.

Tests:

• go test ./internal/glance -run TestStartServerAndReportReturnsStartupError -count=1
• go test ./internal/glance -count=1
• go vet ./internal/glance
• git diff --check

AI disclosure: I used Codex to help investigate, implement, and test this change. I reviewed the final diff and test results.