fix: preserve EXIF timezone offset in JPEG export

[marceldenzer]

Summary

• Bug 1 fixed: DateTimeOriginal was written without inline timezone offset (e.g. 2026-06-05 11:33:26 instead of 2026:06:05 11:33:26+02:00), causing Apple Photos and other viewers to misinterpret the time as UTC
• Bug 2 fixed: Exported JPEG file modification time was +2h — get_creation_date_from_path treated local camera time as UTC before calling set_file_times
• OffsetTime tags added: All three write branches now write OffsetTime, OffsetTimeOriginal, and OffsetTimeDigitized tags for maximum compatibility

Changes in src-tauri/src/exif_processing.rs

• combine_datetime_with_offset: new helper that normalises the date separator (dashes → colons) and appends the timezone offset inline per EXIF 2.31 spec
• parse_offset_secs: new helper that converts +HH:MM / -HH:MM strings to seconds east of UTC; strips surrounding quotes from kamadak-exif display_value() output to prevent silent parse failures
• get_creation_date_from_path (sidecar branch): reads OffsetTimeOriginal/OffsetTime from the sidecar map and subtracts the offset before constructing DateTime<Utc>; falls back to reading the offset directly from the RAW file via rawler when the sidecar predates OffsetTime support
• get_creation_date_from_path (rawler branch): same UTC conversion applied
• write_image_with_metadata (all three branches — sidecar map, kamadak-exif, rawler): DateTimeOriginal and CreateDate now include the inline offset; all three OffsetTime* tags are written

Test plan

• Export a JPEG from an ARW/RAW source with a known timezone offset
• Verify DateTimeOriginal in exported JPEG contains inline offset (e.g. 2026:06:05 11:33:26+02:00)
• Verify OffsetTimeOriginal / OffsetTime / OffsetTimeDigitized tags are present
• Verify file modification date/time matches capture time (not offset by the UTC difference)
• Import exported JPEG into Apple Photos and confirm displayed time is correct
• Test with a sidecar created before this fix (no OffsetTime key) to confirm rawler fallback works

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