ui: surface per-file encoding and line-ending in the values sticky strip and preserve them on save

[bosiakov]

• Added service.DetectFileEncoding that BOM sniffs UTF-8, UTF-8 BOM, UTF-16 LE, UTF-16 BE and scans for the first newline to report LF or CRLF
• Added service.EncodeForFile that prepends the original BOM and rewrites LF to CRLF when writing back, so a UTF-8 BOM CRLF file round trips unchanged
• Extended domain.ValuesFile and service.FlatValues with Encoding and LineEnding fields populated at read time and copied through newFlatValues
• ReadCustomValues and ReadAndMergeCustomValues now detect the encoding pair from the raw bytes; multi file merge takes them from the first file, parallel to indent
• SaveValuesFile and the controller saveToFile accept encoding and line ending parameters; columnFileEncoding pulls them from the column so saves preserve the source shape
• Promoted anyColumnHasCustomValues to exported AnyColumnHasCustomValues so the page layer can gate on a real load
• layoutStickyDiagnostics returns empty dimensions until at least one column has parsed values, then renders override count, extras count, and one encoding chip per loaded column
• Added layoutStickyDiagDivider that draws a hairline vertical separator between adjacent diagnostic chips, and appendDiagChip that skips empty entries so leading and trailing dividers never appear
• Removed the leading plus glyph from the extras count label
• Added constants for the encoding labels, divider dimensions, and the maximum chip count, plus service/file_metadata_test.go covering the detection helper