Replace the two-button captions/transcript switch with a segmented control

[maboa]

Summary

Replace the two-button captions/transcript switch in the navbar with a single
segmented control ("Transcript | Captions") whose active segment is highlighted.

Problem

#caption-editor-btn and #transcript-editor-btn are a mutually exclusive
mode switch implemented as two separate action buttons: clicking one disables
itself and enables the other (js/editor-core.js click handlers). The
disabled button is therefore the only indicator of the current view — but
disabled conventionally reads as "unavailable", not "you are here". The
tooltips name actions ("Edit captions"), not state, so nothing in the UI says
which editor is active.

Proposal

• One two-segment control in the existing design language (daisyUI join or
  tabs tabs-boxed): Transcript | Captions, selected segment highlighted.
• Same behaviour contract as today so programmatic callers keep working — the
  transcription services call
  document.querySelector('#transcript-editor-btn')?.click() on completion
  (deepgram/assemblyai/whisper/parakeet modules), so either keep the ids on
  the segments or keep hidden compatibility elements.
• Slightly narrower than two btn-square buttons — frees navbar space.
• Accessibility: role="tablist"/aria-selected (or a radio group) makes the
  state machine explicit for screen readers, continuing the Accessibility: fix Lighthouse findings (labels, ARIA tab roles, contrast) #402 work.

Notes

Downstream, Glider (the native macOS wrapper) plans to reposition the switch
within its top bar; that's a Glider-side layout concern. The control-form fix
belongs upstream so browser users get the clearer state indication too.