v0.5.1 — read-aloud crash fix & floating overlays

Bug-fix release.

Fixes

• Read-aloud no longer quits the app. Cloud (Deepgram / OpenAI) read-aloud crashed the whole app when playback finished on its own — the AVAudioPlayer was released while its finish callback was still pending on the run loop. It's now always stopped before release. (#41)
• Floating overlays — no more dark frame. The recording pill, read-aloud popup, and clipboard toast now render as rounded pills that float free of the backing rectangle (non-opaque window + clipped, rounded layer + drop shadow), instead of a dark box around the pill. (#42)
• Docs: refreshed README for v0.5.0 features + a new CONTRIBUTING guide. (#40)

Quality

Clean build, 101 tests passing, clippy clean.

Full changelog: v0.5.0...v0.5.1

What's Changed

• docs: refresh README for v0.5.0 + add CONTRIBUTING by @joeVenner in #40
• fix(tts): stop AVAudioPlayer before release — fixes app quitting when read-aloud finishes by @joeVenner in #41
• feat(app): float the overlays as rounded pills (no dark backing frame) by @joeVenner in #42
• chore(release): v0.5.1 by @joeVenner in #43

Full Changelog: v0.5.0...v0.5.1