feat(editor): integrate Key23 keycap design system & keyboard timeline track

Motivation

I use Cap daily for screen recordings. The current keyboard shortcut display shows plain text in a rounded box — functional but visually flat. This PR integrates Key23, an open-source keycap design system I maintain, to offer themed keycap styles in the editor.

What this PR does

Frontend (SolidJS/TypeScript):

• Adds a Keyboard tab in the editor sidebar for configuring keycap appearance
• Adds 5 keycap styles (minimal, apple, pbt, retro, m0116) and 6 color themes
• Adds a keyboard-overlay component that renders styled keycaps on the video canvas
• Adds a KeyboardTrack in the timeline for managing keyboard shortcut segments
• 2D/3D toggle controls keycap depth rendering

Backend (Rust):

• Extends KeyboardSettings in configuration.rs with style, theme, keycap_mode, show_chassis
• Adds theme-aware color mapping in keyboard.rs so exported videos use matching colors

Known limitations & Future work

• Preview vs Export parity: The editor preview renders SVG keycaps (with depth, gradients, etc.), while the Rust native video renderer uses rounded rectangles with matching theme colors. Visual theme/color is consistent, but full SVG rendering in the wgpu pipeline is left as a future iteration.
• Keyboard track is additive: Extends the existing keyboard layer without modifying or removing core behavior.

Testing done

• Verified keycap styles render correctly in editor preview
• Verified theme colors apply in both preview and sidebar config
• Verified 2D/3D toggle works across all render locations
• Verified shortcut parsing handles modifier symbols (⌘T → ⌘ + T)
• Verified Ctrl/Cmd-click multi-selection works on keyboard segments
• Verified with biome check — 0 errors
• Verified with cargo check on crates/project

Related Issue: #2409