refactor: extract copy-mode state machine and clean up session view (#29)

* refactor: extract copy-mode state machine into dedicated module

* refactor: deduplicate diff elements in Edit and ApplyPatch

* refactor: remove as-any casts from part component spreads

* fix: revert rows to plain function to fix copy-mode cursor

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Co-authored-by: leohenon <[email protected]>

Author: leohenon