[bardesss]

List screen and recently read fixes. Stacked on #38; merge that first.

Recently read

• Marking an item unread clears its read time, so a later mark-all does not bring it back into recently read at the old time.
• The prune keep-list counts only read rows, with the same id tie-break as the view, so the view shows the configured number.

List

• Snapshot writes run under a mutex and only for the view that is still showing. A refresh whose write was refused runs again for the new view.
• Mark all read uses the loaded rows only when the stored list belongs to the view being marked.
• Toggling read decides from the database when the operation runs, so two quick taps cancel out.
• The sheet's kept row lives in the view model, so a sheet for a row that dropped out survives rotation. If the row cannot be found at all, the sheet id is cleared.
• A restored search keeps its scroll position. Only a new query scrolls to the top.
• Scroll marking ignores rows kept above the viewport when finding the first visible row, so a row held open above the screen no longer blocks marking.

Links

• openLink returns whether an app opened the link, trims the URL and normalises the scheme (HTTPS:// works).
• A row is marked read only when its link opened. Otherwise a snackbar says "No app can open this link".
• The sheet shows Open only for web URLs.

Tests: ItemDaoTest, ListViewModelTest, MarkQueueTest, new ListScreenStateTest, LinksTest. Full unit tests and lint pass.