CMM-953 fix reader subscribe when logged out

[adalpari]

Description

This PR is showing an informative message when the user is logged out and try to subscribe to a blog.
In addition to the message, we offer the option to open the login flow

Testing instructions

Test Case 1: Discovery Reader - Recommended Blogs

1. Log into the app with a self-hosted page
2. Navigate to Reader → Discover tab
3. Scroll until you see "Recommended Blogs" section
4. Tap the Subscribe/Follow button on any recommended blog
5. Expected: A snackbar appears with message "Log in to follow this site" and a "Log In" action button
6. Tap the Log In button
7. Expected: The sign-in screen opens
8. Log in
9. Verify you can now subscribe

Test Case 2: Post Detail - Menu Subscribe

1. Log into the app with a self-hosted page
2. Navigate to Reader → Discover tab
3. Tap on any post to open the post detail view
4. Tap the three-dot menu (⋮) in the top right
5. Tap Subscribe option
6. Expected: A snackbar appears with message "Log in to follow this site" and a "Log In" action button
7. Tap the Log In button
8. Expected: The sign-in screen opens
9. Log in
10. Verify you can now subscribe

Screen_recording_20251126_175540.mp4

Screen_recording_20251126_175826.mp4

[dangermattic]

	2 Warnings
⚠️	strings.xml files should only be updated on release branches, when the translations are downloaded by our automation.
⚠️	PR is not assigned to a milestone.

Generated by 🚫 Danger

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack by scanning the QR code below to install the corresponding build.

	App Name	Jetpack
	Flavor	Jalapeno
	Build Type	Debug
	Version	pr22370-7610fbd
	Commit	7610fbd
	Direct Download	jetpack-prototype-build-pr22370-7610fbd.apk

Note: Google Login is not supported on these builds.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress by scanning the QR code below to install the corresponding build.

	App Name	WordPress
	Flavor	Jalapeno
	Build Type	Debug
	Version	pr22370-7610fbd
	Commit	7610fbd
	Direct Download	wordpress-prototype-build-pr22370-7610fbd.apk

Note: Google Login is not supported on these builds.

[dcalhoun]

Looks good! Testing worked as expected for me.

Improvement idea: if it's not too complex, would it make sense to subscribe the user to the targeted blog after they finish authenticating? If so, we might show a toast message with the ability to undo.

[adalpari]

Looks good! Testing worked as expected for me.

Improvement idea: if it's not too complex, would it make sense to subscribe the user to the targeted blog after they finish authenticating? If so, we might show a toast message with the ability to undo.

That's a good improvement!
Log in flow does not contain any redirection mechanism as we are just opening the generic login screen. Definitely, we should implement something like that if this ends up being a common user case. Otherwise, I don't think it's used enough right now.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 8.33333% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 39.02%. Comparing base (889668d) to head (7610fbd).
⚠️ Report is 2 commits behind head on trunk.

Files with missing lines	Patch %	Lines
...ui/reader/discover/ReaderPostCardActionsHandler.kt	9.09%	9 Missing and 1 partial ⚠️
...droid/ui/reader/discover/ReaderNavigationEvents.kt	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            trunk   #22370      +/-   ##
==========================================
- Coverage   39.03%   39.02%   -0.01%     
==========================================
  Files        2203     2203              
  Lines      106328   106340      +12     
  Branches    15059    15061       +2     
==========================================
+ Hits        41500    41501       +1     
- Misses      61338    61348      +10     
- Partials     3490     3491       +1     

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