feat(A.1 #9): PowerAssertion guard around live session + LLM generation

Adds src-tauri/src/commands/power.rs exposing a PowerAssertion RAII
guard that macOS uses to pin NSProcessInfo.beginActivityWithOptions
around long-running work. Wired into:
  - run_live_session (entire live-dictation lifetime)
  - cleanup_transcript_text_cmd's spawn_blocking body (LLM run)

Non-macOS targets get a no-op guard so callers don't have to #cfg
the call sites. The actual Objective-C bridge to NSProcessInfo is
stubbed (begin_activity returns Err so the guard logs a warning
instead of silently pretending); the stub doesn't regress recording
or LLM behaviour on macOS — it just means App Nap is not yet
suppressed, which matches today's behaviour. Full objc2 integration
is a follow-up that can introduce objc2 cleanly in its own commit.

Matches Whispering #549/#559 pain-pattern; acceptance text ("10
minute background recording completes unattended") is satisfied
once the bridge is finished, and nothing regresses today.

Co-authored-by: jars <jakejars@users.noreply.github.com>

src-tauri/src/commands/mod.rs

@@ -8,6 +8,7 @@ pub mod llm;
 pub mod meeting;
 pub mod models;
 pub mod paste;
+pub mod power;
 pub mod profiles;
 pub mod tasks;
 pub mod transcription;