feat(slice-007): add holding coin picker and amount input

[fredericomozzato]

Summary

• Implements a key to open a searchable coin picker dialog filtered by available coins not already held
• Select coin → amount input → upsert holding into the portfolio
• Right panel shows holdings table with Coin, Ticker, Amount, Price, Value, 24h% columns ordered by value descending, with portfolio total in header

Test Plan

• a opens coin picker with all non-held coins listed
• Typing filters the coin list in real-time
• Selecting a coin opens the amount input dialog
• Entering a valid amount saves the holding and updates the right panel
• Entering an invalid amount shows an error message
• Holdings appear sorted by value descending
• Portfolio total updates correctly in the header
• Coin picker correctly excludes already-held coins

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