feat(TG-1006): add periodic feedback widget with Slack notifications

[FernandezAstor]

• Adds emoji rating feedback form rendered as a fixed bottom-right VanJS component
• Popup appears once every 30 days per user; timestamp stored in auth_users.preferences JSONB
• TG_DISABLE_FEEDBACK_POPUP env var suppresses periodic popup for enterprise customers
• "Give Feedback" entry added to Help menu for on-demand access
• Submitted feedback sent to MixPanel; optional Slack webhook via TG_SLACK_FEEDBACK_WEBHOOK
• DB migration 0180 adds preferences column to auth_users

Note: popup eligibility check runs in Router.run() (inside cookies_ready block) instead of Router._init_session() — cookies are not available when _init_session runs.

[github-actions]

Coverage Report •

File	Stmts	Miss	Branch	BrPart	Cover	Missing
testgen/common/models
user.py	48	18	4	0	58%	31–43, 46–47, 52–53, 57–58, 62–63, 67–68
testgen/ui
session.py	54	19	8	0	56%	49–52, 55, 58–60, 63–65, 69–79
testgen/ui/components/widgets
page.py	102	76	18	0	22%	23–39, 43–70, 91, 95, 99, 103, 107, 111, 115, 119, 123, 127–130, 145–185
testgen_component.py	27	19	12	0	21%	48–78
testgen/ui/navigation
router.py	121	98	28	0	15%	26–27, 31–40, 43–69, 72–120, 123, 130–132, 135, 138–166, 169–176, 179–185
TOTAL	13072	9912	3060	2	20%

Tests	Skipped	Failures	Errors	Time
5	0 💤	0 ❌	5 🔥	12.906s ⏱️