refactor: remove in-app feedback panel, link Help menu to GitHub Issues

[datlechin]

Summary

• Deletes the in-app Help > Report an Issue panel (FeedbackView, FeedbackViewModel, FeedbackAPIClient, FeedbackDiagnosticsCollector, FeedbackDraft, FeedbackWindowController — 916 LOC) along with its docs page and screenshots.
• Replaces the menu item with a one-liner that opens https://github.com/TableProApp/TablePro/issues via NSWorkspace.shared.open. GitHub's bug/feature templates already cover what the in-app form did.
• Drops 8 feedback-only localization keys (Bug Report, Feature Request, General Feedback, the network/server/rate-limit/too-large/unexpected-response error strings, and the "Report an Issue..." ellipsis label). Keeps the shared Report an Issue, Network error: %@, and Server error (%d): %@ keys used elsewhere.
• The api.tablepro.app/v1/feedback server endpoint is no longer called by any client and can be retired separately.

Test plan

• Build clean in Xcode (-skipPackagePluginValidation).
• Launch app: Help menu shows Report an Issue (no ellipsis).
• Click it: default browser opens https://github.com/TableProApp/TablePro/issues.
• App launches with no console warnings about FeedbackAPIClient / FeedbackViewModel symbols.
• swiftlint lint --strict passes.
• grep -rn "Feedback" TablePro/ --include="*.swift" shows no feature symbols (only the unrelated draggingDestinationFeedbackStyle and copyQueryWithFeedback matches).