v3.5.1: OTA download fix, reworked update result screens, timeout slider

Fixes OTA downloads (GitHub's asset redirect wasn't followed before,
so every download failed), reworks the update success/failure
screens to require explicit confirmation instead of auto-vanishing
or auto-restarting, and drops the release-asset rename step in favor
of just uploading firmware.bin directly.

Replaces the screen-timeout tap-cycling with a drag slider (1-15
minutes, then "Never"); the idle screensaver toggle moves into this
same screen since it only affects what happens when the timeout
fires.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/config.h

@@ -6,7 +6,7 @@ namespace Config {
     // CLAUDE.md-Workflow "Standard-Workflow: Push & Release") - erscheint
     // im Info-Screen (Menue > System > Info) und muss zum jeweiligen
     // Git-Tag passen.
-    constexpr const char* APP_VERSION = "3.5.0";
+    constexpr const char* APP_VERSION = "3.5.1";
 
     // Display-Helligkeit (Menue > System > Helligkeit), in Prozent.
     // MIN bewusst nicht 0 - ein komplett dunkles Display koennte sonst wie