Hide empty live transcript tab

[ComputelessComputer]

Only show the transcript tab when transcript/audio/live/batch evidence exists and add active-meeting regression coverage.

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Note

Low Risk
UI-only tab gating in the session editor with focused unit tests; no auth, data, or persistence changes.

Overview
Hides the session editor Transcript tab when there is nothing to show, especially at the start of an active meeting where recording may exist but no transcript, live segments, or batch work is present yet.

Tab visibility now goes through useCanShowTranscript, which still allows the tab when there are saved transcript rows, in-flight batch transcription, batch errors, or live segments for the current session. Saved audio alone no longer qualifies while the session is in active or finalizing live capture; for inactive sessions, audio without transcript rows can still surface the tab (e.g. pending batch). useEditorTabs and computeCurrentNoteTab both respect that flag so users are not dropped onto an empty transcript view.

Adds regression tests in the note-input header suite for active meetings with/without live segments and for inactive sessions without evidence.

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[cursor]

Cursor Bugbot has reviewed your changes using default effort and found 1 potential issue.

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[ComputelessComputer]

Closing because this patch is already represented on main.