Add parameter tooltips to stage controls

[OpenSauce]

Context

Stage parameters (gain, threshold, ratio, etc.) are labelled but offer no explanation of what they do or how they affect the sound. New users — especially those unfamiliar with amp/effects terminology — have to guess or look things up externally.

Iced provides a Tooltip widget that can wrap any element and show text on hover, so the implementation cost is low.

Scope

• Stage parameter knobs/sliders — short description of what the parameter controls and how changing it affects the sound (e.g. "Controls the amount of signal driven into the preamp tube model. Higher values add more distortion and harmonic content.").
• Global controls — oversampling selector, IR cabinet loader, tuner, recorder, etc., where a one-liner would help.
• Tooltips must go through tr!() so they work in both EN and ZH_CN.

Acceptance Criteria

• Hovering over any stage parameter control shows a tooltip describing what it does
• Tooltip text is localised via tr!() (EN + ZH_CN keys added to src/i18n/mod.rs)
• Tooltips on global/IO controls (oversampling, IR loader, tuner, recorder) where useful
• No tooltip appears when interacting with (dragging) a knob — only on idle hover
• Tooltip positioning doesn't clip off-screen on edge controls