Activation UX: add a real "talk" affordance (button + optional hold-to-talk hotkey)

[mabry1985]

Context

Spun out of the activation-UX pass that disabled wake word and renamed "Push-to-talk" → "Tap to talk" (PR linked below). The rename made the label honest, but the underlying affordance gap remains.

Problem

What we call the default activation style is really double-click the orb to toggle a listening turn (web/src/plugins/orb/OrbStage.tsx:51 → invoke('set_mic_listening', { on })). Concretely:

• It's a toggle, not press-and-hold (double-click starts a turn, double-click again stops).
• There is no dedicated talk button anywhere. The only persistent control is the chrome-rail mic button, which is hard mute/unmute — a different axis.
• There is no keyboard shortcut to talk.
• The behavior is only explained by a settings hint ("Double-click the orb to talk"), so it's effectively undiscoverable for a new user.

Proposal (B)

Give "talk" a real, discoverable affordance:

1. Visible talk button in the chrome rail (next to the mic/mute button) that starts/stops a listening turn — reuses the existing set_mic_listening Tauri command, so it's a UI + wiring change, not new audio plumbing.
2. Optional global hold-to-talk hotkey (press-and-hold to open the mic, release to close) — this is what would make "push-to-talk" literally true, for users who want walkie-talkie behavior. Likely a Tauri global shortcut + a held-vs-toggle mode.
3. Decide whether to surface hold-to-talk as a distinct activation style or a modifier on "Tap to talk."

Out of scope / related

• Possible larger rethink of the three activation styles (tap-to-talk / open-mic / wake-word) — collapse or simplify (direction "C" from the discussion). Can fold in here or split.
• Wake word stays disabled until the retrained model ships (web/src/shared/wakeword/enabled.ts).

Acceptance

• A first-time user can see how to start talking without reading settings.
• Push-to-talk (hold) works via keyboard for users who enable it.