feat: Add live microphone oscilloscope during recording

[Butenkite]

Summary

Adds a MicWaveform canvas component that renders a real-time oscilloscope from the existing interview AnalyserNode time-domain data during the recording stage. InterviewClient wires the component into the current silence-detection animation frame loop so users can see whether the microphone is idle or receiving speech, using the same RMS threshold as VAD. Storybook stories and stub type updates support the new component without changing backend behavior or environment configuration.

Components touched

Component / surface	What changed	Visible when
MicWaveform (new)	Canvas oscilloscope with idle vs speaking states driven by RMS threshold	Recording stage
InterviewClient (modified)	Renders MicWaveform and calls drawWaveform from the existing analyser loop	Recording stage
Storybook MicWaveform stories (new)	Silent, active speech, inactive, and paint-helper demos	Storybook only
storybook.d.ts (modified)	Adds decorators and render to stub Storybook types	Typecheck only

Drawing runs imperatively on canvas inside the existing requestAnimationFrame loop — no per-frame React state.

File-by-file changes

Frontend components

• frontend/app/InterviewClient.tsx: Imports MicWaveform, adds a ref, calls drawWaveform in the existing analyser loop, and renders the waveform when stage === "recording" so users get live mic feedback during answers.
• frontend/app/components/MicWaveform/MicWaveform.tsx: New forward-ref component with exported paintMicWaveform helper, canvas resize handling, and imperative drawing API needed to visualize analyser samples without re-rendering React each frame.
• frontend/app/components/MicWaveform/MicWaveform.stories.tsx: New Storybook stories with mock silent and animated speech demos so the oscilloscope states can be reviewed outside the live interview flow.

Frontend styles

• frontend/app/components/MicWaveform/MicWaveform.module.css: New module styles for the waveform container, canvas frame, and idle vs speaking status labels required by the component conventions.

Types / interfaces

• frontend/types/storybook.d.ts: Extends the local Storybook stub with decorators and render so the new MicWaveform stories typecheck correctly.