feat: expose voice input spike

[SebastianBoehler]

Summary

• Replaces the placeholder /voice route with a voice input spike screen.
• Routes Ask FAB long-press to the voice screen while keeping normal tap behavior for chat.
• Surfaces push-to-talk, conversation mode, custom hotword, and passive listener feasibility gates in-app.

Stacked after #57. Closes #50

Validation

• git diff --check
• flutter test test/navigation_test.dart could not run in this runtime because flutter is not installed (command not found).

[SebastianBoehler]

Post-merge review pass on #58.

Intent I see here:

• Replace the placeholder /voice route with a visible voice input spike screen.
• Make the Ask FAB long-press route to /voice while preserving normal tap-to-chat behavior.
• Keep the voice feature framed as feasibility work, especially around push-to-talk vs. conversation mode vs. hotword/passive listening.

I did not find a concrete blocking defect in the diff. The useful risk areas for the next pass are:

• Real-device voice behavior: mic permission denial, unsupported speech engines, and STT/TTS lifecycle are still the highest-risk parts and need Android/iOS/manual validation beyond widget tests.
• Ask FAB interaction/accessibility: the switch from FloatingActionButton.extended to a custom GestureDetector/Material wrapper enables long-press, but it is worth checking keyboard/focus/tooltip/ink behavior on web and desktop so the primary Ask action stays accessible.
• Discoverability: long-press is a hidden gesture, so the next UX iteration should decide whether voice needs a visible entry point as the spike becomes more than a prototype.
• Voice flow scope: keep passive listening/hotword behind explicit research gates; the current copy points in the right direction.

Validation checked:

• GitHub Actions for this PR passed Flutter formatting, analysis, tests, Android APK, and desktop/web builds.
• Local git diff --check origin/main..pr-58 produced no whitespace errors.
• I could not rerun flutter test locally because this runtime does not have flutter installed.

Smallest next action: no follow-up PR needed from this review alone. If someone continues the voice work, I would start with a manual device test checklist and a small accessibility pass on AskFab before adding more voice features.