Add DCA support for non-XCH base assets

Allow DCA bots to use selectable base/spend assets, not only XCH, so users can create bots like wUSDC.b → XCH or other CAT/base combinations.

Suggested controls:

• Spend asset dropdown

• Target asset dropdown

• Percentage of selected wallet asset

• USD-equivalent mode for stable assets

• Take-profit / sell-back rules

• Clear warning when pair liquidity is thin