fix: show available margin in edit margin modal

[autochamchikim-pixel]

Summary

• Shows an Available row in the Add/Remove Margin modal.
• For Add mode, the row uses the connected wallet balance for the position asset.
• For Remove mode, the row uses the removable position margin plus funding.
• The available value is clickable and fills the amount input, matching the existing pool deposit/withdraw UX.

Test plan

• node -e "const fs=require('fs'); const svelte=require('svelte/compiler'); svelte.compile(fs.readFileSync('src/components/modals/EditMargin.svelte','utf8'), {filename:'src/components/modals/EditMargin.svelte'}); console.log('svelte_compile_ok')"
• Attempted full npm run build; the container killed Rollup with exit 137 after dependency install, likely memory-related, before surfacing code errors.

Closes #19

[autochamchikim-pixel]

Requesting assignment/consideration for #19. I opened this PR to resolve the bounty: #141

[autochamchikim-pixel]

Pushed a small follow-up quality pass: zero balances now render as 0 instead of -, the available value only fills the input when loaded, and ERC20 approval is gated to Add mode so Remove mode keeps showing the remove action.