fix(dex): resolve precision loss in wallet

[CharlVS]

Fix precision loss in fraction parsing and add an "Exact" taker amount option to resolve order matching issues for large volumes.

The fract2rat utility previously converted fraction numerator/denominator via double.parse before converting to BigInt, which introduced precision loss for large values. This caused mismatches, particularly when a maker's min_vol was exactly equal to max_vol, preventing takers from matching these orders precisely. The "Exact" button provides a direct way to use the maker's precise maxVolume without UI-induced rounding.

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[cursor]

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[github-actions]

Visit the preview URL for this PR (updated for commit 8f1dbff):

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[smk762]

• Ran 5 test swaps where min == max, matched with exact, using values with > 10 significant figures `. All 5 completed successfully ✔️
• confirmed min/max validation working in form
• confirmed new "exact" button functioning as expected.

LGTM to approve, thanks!

A minor enhancement which I can send to issue would be some form of visual indication where an order is exact only ( e.g. 🎯 ) though this should be discussed more to cover other min_volume != null cases