feat(hotkey): add configurable LLM invert modifier for recording

[thedavidweng]

Summary

Implements #15 by adding a configurable modifier key that temporarily
inverts whether the current recording session uses LLM polishing.

The inversion is evaluated when recording starts, applies only to that
session, and does not mutate the persisted llm.enabled setting.

Changes

• add hotkey.llm_invert_modifier to Rust config with default control
• support control | option | command | shift | fn | none and normalize
  invalid values back to control
• expose the invert modifier flag through SPHotkeyConfig
• extend the session begin context so ObjC can pass the per-session LLM
  inversion state into Rust
• capture the configured modifier in the hotkey monitor and lock the
  inversion state when recording begins
• apply session-level LLM decision logic in koe-core without writing
  back to config.yaml
• add an LLM Modifier setting in the Hotkey / Controls UI
• update README config examples and behavior docs

Behavior

• normal recording continues to follow llm.enabled
• pressing the configured modifier while starting recording flips the
  LLM behavior for that recording only
• when llm.enabled = true, the inverted session skips LLM polishing
• when llm.enabled = false, the inverted session attempts to use LLM,
  but still requires a valid active LLM profile
• setting the modifier to none disables inversion behavior entirely

Testing

• cargo test -p koe-core
• make generate
• cargo build --manifest-path koe-core/Cargo.toml --release --target aarch64-apple-darwin
• cd KoeApp && xcodebuild -project Koe.xcodeproj -scheme Koe -configuration Debug ARCHS=arm64 build

Notes

• Xcode build succeeds with the existing linker warnings about some Rust
  object files being built with a newer macOS version marker than the
  app deployment target