feat(sig): Report list toolbar as core control

[Twixes]

Problem

The inbox list toolbar introduced in #1513 added controls only for bulk operations. But the same controls have been missing for individual reports too. In the Cloud version, we had them under a dropdown in the report pane itself. I considered doing the same here, but there's no point when we have a great toolbar - the toolbar should be the individual report controls as well.

At the same time, checkboxes take away from the content when shown always - and we don't need them by default for most user interactions here. (Ofc will validate this, but at least I think that's a safe assumption for now that the individual report level is the primary one.)

Changes

Making the report list toolbar always-on, with intuitive logic for selection state synced with the currently-viewed-report state. Unifies the selection state, so that "currently viewed" and "selected" are the same thing. Checkboxes only show up when multiple reports are selected. For multi-select, you can use the "Select all" checkbox or standard desktop Shift/Cmd+click patterns. See in action:

[Twixes]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat(sig): Report list toolbar as core control #1532 👈 (View in Graphite)
• feat(sig): Inbox toolbar disabled clarity #1531
• style(sig): Inbox list chrome cleanup #1530
• feat(sig): add modifying report state via inbox #1513 : 1 other dependent PR (#1543 )
• feat(sig): break inbox into some components #1505
• main

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