Fix Ctrl shortcuts in step 2/3 viewers (#546)

[michalpelka]

Fixes #546.

Step 2

• Ctrl+R had no visible effect: it changed each cloud's color but left the color mode on the default intensity gradient, so the new colors were never drawn. It now switches the mode to "Random per segment", the same as the menu item.
• View jumped after repeated Ctrl+R: after every Ctrl shortcut, the app faked a Ctrl release. rlImGui only reports Ctrl when it goes down or up, so while Ctrl was still held that fake release was never corrected. The next R counted as a plain R, which switches the camera to the "Right" view. I removed these fake releases from all Ctrl shortcuts in step 2 and step 3.
• Ctrl+R and Ctrl+T now fire once per press instead of repeating while held.

Step 3

• Ctrl+D now opens session removal (previously Ctrl+R). The menu label and shortcut list are updated.
• Ctrl+R gives each session a random color and applies it to that session's clouds and trajectory.

Notes

• In manual loop closure, source and target are still drawn red and blue by default, so Ctrl+R shows there only after that option is turned off.
• The legacy step 3 app is unchanged and still uses Ctrl+R to remove sessions.

Test plan

• Step 2 and step 3 build (macOS, release)
• Step 2: hold Ctrl and tap R several times; colors change and the camera stays put
• Step 3: Ctrl+D opens session removal, Ctrl+R randomizes session colors
• After closing the file dialogs from Ctrl+O and Ctrl+S, plain view keys (R, T, ...) still work

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