Polish chat custom modes

[roblourens]

• Fix bugs: New Chat Mode Cannot Act as Agent Mode – Tools Unavailable #251313
• Properly support adding/removing/editing modes
• Support custom keybindings for modes, and ux for editing these

[wail-asad]

Dear @roblourens,
Could you please confirm if this is similar to issue #251251? If so, kindly add the LLM model temperature parameter to this and close #251251.
Thank you.

[aeschli]

#251251 is not covered by this issue.
Allowing modes to affect the temperature or other model properties is not planed yet

[aeschli]

Fixed #251313

Custom keybindings have not yet been added.