[Linux] Remove LinuxInputMethodBootstrap after JavaFX dead-key fix validation

[remiceres]

Why

We currently keep a Linux-only IME relaunch workaround for dead keys / accent composition.

Code:

• LinuxInputMethodBootstrap.java
• call site in App.java

Upstream tracking:

• dead-key bug: https://bugs.openjdk.org/browse/JDK-8367045
• JavaFX 25 release notes: https://gluonhq.com/products/javafx/openjfx-25-release-notes/
• jfx25u backport discussion: https://www.mail-archive.com/openjfx-dev%40openjdk.org/msg24505.html

Goal

Drop the Linux relaunch workaround and keep a single startup path.

Plan

• Upgrade JavaFX to a build that includes the Linux dead-key fix.
• Test on Linux (GNOME/IBus + KDE/Fcitx if available):
  • WebView editor input
  • standard JavaFX text fields
  • dead keys / accent composition
• Remove:
  • LinuxInputMethodBootstrap.java
  • bootstrap call in App.main
  • related env-var docs (CORESE_IM_BOOTSTRAP_*)

Done when

• No bootstrap/relaunch code remains.
• No CORESE_IM_BOOTSTRAP_* docs remain.
• Linux accent/dead-key input works without GTK_IM_MODULE=xim / XMODIFIERS=@im=none.
• No regressions on splash/startup/Ctrl+C behavior.