Enable transcript speaker reassignment

[ComputelessComputer]

Add clickable speaker labels with all-matching and segment-only assignment modes.

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Note

Medium Risk
Changes speaker identity persistence, live reconciliation, and Rust segment rendering together; incorrect scope or conflict logic could mislabel speakers, though coverage is extensive in unit tests.

Overview
Adds segment-only speaker reassignment alongside the existing all matching (channel/speaker) flow, so users can correct a single transcript segment without relabeling every word from that diarization cluster.

The SpeakerAssignPopover now exposes an All matching / This segment toggle (defaulting back to All matching when the popover closes) and passes the selected mode plus the segment’s persisted word ids into upsertSpeakerAssignment. The previous DirectMic read-only label path is removed so self-channel labels use the same assign UI.

Persistence and live STT store segment overrides as user_speaker_assignment hints with scope: "segment" and word_ids, with updated conflict rules between full-speaker and word-scoped hints. The transcript accumulator reconciles those hints when words are replaced or when more speech continues the same speaker segment.

Rendering maps segment hints to IdentityScope::Words for Rust segment building, which now applies word-level human overrides so only selected words split into different speaker segments while the rest keep the channel/speaker assignment.

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