test: complete documented browser and device checks for M4

[robertky]

Run the final manual quality check after the other M4 changes are merged. This records the checks automated tests cannot cover.

Tasks

• Check the deployed prototype or local production preview in one desktop browser and one narrow mobile-sized viewport
• Verify subject selection, sorting, previous/next navigation, jump-to-book, adding/removing favorites, and saved choices
• Check keyboard-only use: tab order, visible focus, selects, buttons, and removing a favorite
• Check that loading, error, and no-results messages are understandable
• Add a closing comment with the browsers/viewports checked and the result
• Create a separate bug issue for anything that cannot be fixed immediately

Acceptance criteria

• The complete M4 user flow has been checked manually on desktop and mobile
• Any discovered defects are either fixed or tracked in their own issue

No implementation branch is needed unless this check uncovers a fix. Part of M4.

[rachelks]

Final M4 manual quality check completed using the local production preview.

Tested the complete library flow on desktop and a narrow mobile-sized viewport, including subject selection, sorting, Previous/Next navigation, jump-to-book, favorites, saved choices, keyboard-only use, visible focus, and responsive layout.

Loading and error states were also checked manually using network throttling/offline mode. The no-results state is covered by the automated tests but could not be triggered through the available subject options during the manual check.

No issues were found during the manual check.