Paka 0.15.0 capture preview 3.1

Point release on top of capture preview 3: a battery/lifecycle pass plus a Kotlin update.

• Background work now pauses instead of relying on Compose keeping effects alive: the per-second 2FA code tick and the capture screen's focus-retry loop stop while Paka is backgrounded
• An open PDF releases its decrypted session and native memory when backgrounded (like photos already did) and reopens on return
• Stack previews recycle their rendered PDF page bitmaps instead of leaking them until garbage collection
• The scanner and capture light label no longer reads "on" after returning from the background, where the torch was actually switched off
• Kotlin updated to 2.4.0 (dependency-verification checksums regenerated); no behavior change

Worth testing on the device: leave the codes screen and the camera open, background and return, and confirm the light label and focus behave; open a PDF, background, return, and confirm it reappears cleanly.

Signed like the regular releases, so it installs as an update over an existing Paka installation (including earlier previews). Source is on the preview/document-capture branch; main remains at 0.14.0.