Fix sync status and diff handling for branch/subfolder text files

[lukelzlz]

What changed

• preserve the configured branch when fetching remote file contents during sync
• mark sync results as failed when pull operations return file errors
• resolve diff-view remote paths inside configured repository subfolders
• auto-detect common East Asian text encodings when decoding remote GitHub file contents

Why

Sync and diff behavior could be misleading in a few important cases:

• branch-specific files could be fetched from the default branch instead of the configured branch
• sync logs could report completion even when remote pulls failed
• diff view could request the wrong remote path when the vault mapped into a repo subfolder
• non-UTF-8 remote notes could render as mojibake in diff view

Impact

• sync status is now more accurate
• diff view shows the correct remote file in subfolder-based setups
• remote text previews handle more real-world note encodings

Validation

• npm test
• npm run build