Migrate Flatpak runtime from EOL org.kde.Platform 5.15-24.08 to 24.08

## Summary

The Flatpak build of Speech Note currently uses the end-of-life runtime `org.kde.Platform 5.15-24.08`, which is no longer receiving fixes or security updates.

## Current State

- **Runtime:** `org.kde.Platform/x86_64/5.15-24.08`
- **Application version:** 4.8.4 (commit dated 2026-04-17)
- **Status:** EOL (per `flatpak upgrade` output)

```
Info: runtime org.kde.Platform branch 5.15-24.08 is end-of-life, with reason:
We strongly recommend moving to the latest stable version of the Platform and SDK
Info: applications using this runtime:
net.mkiol.SpeechNote
```

## Request

Please update the Flatpak manifest to use the latest stable KDE Platform runtime (e.g., `org.kde.Platform/x86_64/24.08` or newer). This ensures continued security patches and alignment with the current Flatpak ecosystem.

If there are specific compatibility blockers preventing migration, it would be helpful to document them so the community can assist.

## Environment

- OS: Linux (Flatpak system installation)
- Flatpak version: 1.16.6
- Speech Note version: 4.8.4
- Affected branches: `stable` (only available branch)

Thank you for maintaining Speech Note — it's a great tool!

Inspect the Flatpak manifest and locate the org.kde.Platform runtime entry. Check the current runtime and SDK requirements, then update the manifest to a current stable KDE Platform branch such as 24.08 or newer. Build or install the Flatpak and confirm that flatpak upgrade no longer reports the runtime as end-of-life.

Written by the indexing model from the issue text.