docs(session-replay): document isSessionReplayActive and pausing replay on sensitive Flutter screens

[lukas-roqqu]

Changes

Documents what is still missing from the Flutter session replay docs after #20094.

The issue overstated the gap: startSessionRecording({resumeCurrent}) and stopSessionRecording() were already documented in the Flutter tab of programmatic start and stop controls, and the installation page links there. What was missing is isSessionReplayActive() (in posthog-flutter since 4.7.0, PostHog/posthog-flutter#123), and any guidance that masking alone is not enough on keypad-style screens.

• session-replay/_snippets/flutter-manual-replay-control.mdx – adds isSessionReplayActive() to the methods list, plus a "Pause recording on a sensitive screen" subsection with a minimal StatefulWidget that calls stopSessionRecording() in initState and startSessionRecording() in dispose. Masking hides the pixels of a PIN, passcode, or card keypad, but the native SDKs still record tap coordinates, so with a known keypad layout the entered value can be reconstructed (feat(replay): allow disabling touches without stopping screenshots posthog-flutter#575 describes this). The existing NavigatorObserver example stays as the route-based alternative.
• session-replay/_snippets/flutter-privacy.mdx – new "Pausing recording on sensitive screens" section in the Flutter tab of /docs/session-replay/privacy, after "Masking in Flutter": masking hides pixels, not touch positions, so stop recording while a keypad screen is shown and resume afterwards. Links to the programmatic controls rather than repeating the example.
• session-replay/_snippets/flutter-installation.mdx – the "Manually control session recordings" paragraph now also mentions isSessionReplayActive().

captureTouches from PostHog/posthog-flutter#575 is unreleased, so it is intentionally not documented here; stop/start is the mitigation available today.

Closes #20094

Checklist

• I've read the docs and/or content style guides.
• Words are spelled using American English
• Use relative URLs for internal links
• I've checked the pages added or changed in the Vercel preview build (reviewer: please check the Flutter tabs on /docs/session-replay/how-to-control-which-sessions-you-record, /docs/session-replay/privacy, and /docs/session-replay/installation/flutter in the preview)
• If I moved a page, I added a redirect in vercel.json (no pages moved)