Split the cancel out of ConnectButton into its own button

[fernandomg]

User story / Problem statement

Currently, ConnectButton cancels the connect when clicked while it reads "Connecting…". A double-click on Connect therefore connects and cancels itself, and a button whose label says one thing and whose click does another is a trap. Raised in this review comment on #142.

Expected outcome

ConnectButton only connects and is inert while pending, as before #142. A new CancelConnectButton only cancels and is inert when nothing is connecting. A double-click on either does nothing past the first click. The prompt in dapp/frontend shows the cancel button beside the connect button instead of the "(click to cancel)" hint.

Acceptance criteria

• ConnectButton is inert while pending, with the disabled look back in canton-theme
• CancelConnectButton ships on the /connect sub-path with anatomy, theme rules, tests and docs
• CancelConnectButton is inert (aria-disabled) when nothing is connecting
• Double-clicking either button has no effect past the first click
• ConnectPrompt renders both buttons and drops the hint

Alternatives considered

• Ignore multi-clicks with event.detail > 1 in composeAction: one line, but one button keeps doing two things
• Arm the cancel after a delay: timing-based and surprising

Technical notes

Keep both buttons mounted and in place while connecting: if the connect button takes the cancel button's spot, the second click of a double-click starts a new connect.