feat: restore feedback modal for user interest collection

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Summary

Restores the feedback modal that was removed in PR #6533 to help identify Continue knowledgeable/friendly users for CAI user interviews.

Changes

• ✨ Restored FeedbackDialog component with modern, improved styling using Tailwind CSS
• 🎨 Enhanced UX with:
  • Clean, card-based layout
  • "Not now" dismiss button for less intrusive experience
  • Auto-close modal after successful submission
  • Better visual hierarchy and spacing
• 📊 Restored tracking logic:
  • LocalStorage counter that tracks user interactions
  • Triggers modal after 300 interactions
  • Captures name and email via PostHog user_interest_form event
• 🔧 State management restored in Redux uiSlice

Testing

The modal will appear after 300 chat interactions. To test manually:

1. Set mainTextEntryCounter in localStorage to 299
2. Send one more chat message
3. Verify the modal appears with improved styling

Related

• Closes: CON-4856
• Related PR: chore: remove feedback dialog #6533 (removal)
• Slack thread: https://continuedev.slack.com/archives/C060AV775UZ/p1762571766597529

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This agent session was co-authored by dallin and Continue.

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Summary by cubic

Restores the feedback modal to collect user interest for CAI interviews. It triggers after 300 chat interactions and submits name/email via PostHog, meeting the requirements in CON-4856.

• New Features
  • Reintroduced FeedbackDialog with modern Tailwind styling, “Not now” dismiss, and auto-close after submit.
  • Tracks interactions via localStorage and triggers the modal at 300; captures details with user_interest_form.
  • Restores dialog state handling in Redux and adds setDialogEntryOn; adds mainTextEntryCounter to localStorage types.

^{Written for commit a258974. Summary will update automatically on new commits.}

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Test Failure Analysis

The macOS test failures in TUIChat.editMessage.test.tsx are unrelated to this PR's changes:

✅ All relevant checks passed:

• lint ✅
• gui-checks ✅
• core-checks ✅
• prettier-check ✅

❌ Failing tests are in CLI (extensions/cli):

• src/ui/__tests__/TUIChat.editMessage.test.tsx
• Only failing on macOS (18, 20, 22) - macOS 24 passed
• All Ubuntu and Windows tests passed

📁 This PR only modifies GUI files:

• gui/src/components/dialogs/FeedbackDialog.tsx (new file)
• gui/src/pages/gui/Chat.tsx
• gui/src/redux/slices/uiSlice.ts
• gui/src/util/localStorage.ts

The test failures appear to be a flaky test issue specific to certain macOS versions, not caused by this PR's changes.