fix: import system error handling, decompression, and correctness

[datlechin]

Summary

• Targeted fixes for the import system (architecture was already sound, unlike export which needed full rewrite)
• 10 issues fixed: 2 critical, 5 major, 3 minor

Changes (9 files, +95/-84)

Critical fixes

• SQLFileParser now returns AsyncThrowingStream instead of AsyncStream - file read and encoding errors are surfaced instead of silently succeeding with partial data
• Compressed (.gz) files are decompressed once and shared between preview and import (was decompressing twice)

Major fixes

• Transaction rollback error now correctly reported (was wrapping import error instead of rollback error)
• FK checks properly restored after failed import with logging (was silently swallowing)
• Progress estimate uses decompressed file size for accurate progress bar
• NSOpenPanel queries import plugins for accepted file types instead of hardcoding .sql/.gz
• FileDecompressor uses async GzipProcess instead of blocking waitUntilExit()

Minor fixes

• Statement counting task cancelled on dialog dismiss
• importFileURL cleared after dialog presented
• Temp file extension derived from original filename (not hardcoded .sql)

Test plan

• Import a .sql.gz file - verify single decompression, correct progress
• Import a file with encoding error - verify error surfaced (not silent success)
• Import with FK checks + transaction, trigger failure mid-import - verify rollback + FK restored
• Cancel import mid-stream - verify clean dismissal
• Reopen import dialog - verify no stale file pre-populated