Export image-based: keep the profile's Resolution (fix #12244)

[niksedk]

Fixes #12244 — profiles in the "Export to … (Blu-ray sup)" dialog lost (or silently overwrote) their Resolution on reopen, while font size restored fine.

Root cause — two compounding defects

1. LoadProfile matched resolution by Width only, against presets only. The save path writes both ScreenWidth and ScreenHeight, but the load matched Resolutions.FirstOrDefault(r => r.Width == profile.ScreenWidth). A profile's custom resolution (UHD 3840×1920) has no preset with that height, so it snapped to the same-width preset (3840×2160) or, with no width match, to the 1920×1080 default. Font size round-trips by exact value — hence only the resolution seemed to "reset".

2. Initialize force-selected the loaded video's resolution after the profile was applied. ExportBluRaySup passes the main window's video in, and Initialize asynchronously set SelectedResolution to the video's size, overriding the profile. Because SaveProfile runs on close, the video's size was then written back into the selected profile — corrupting it. This is what made the reporter's UHD profile display the HD source video's 1920×960.

Fix

• Add EnsureResolutionItem(width, height) — matches on both dimensions and materialises a custom ResolutionItem (just below the "pick from video" entry) when the exact size isn't a preset, so any resolution round-trips.
• LoadProfile uses it, so custom resolutions restore exactly.
• Initialize now only makes the video's resolution available in the combo and falls back to it only when the profile left no valid resolution selected (SelectedResolution ??= item instead of overwrite). A profile with a resolution always wins; the video can still be picked manually.

Verification

Standalone harness reproducing the reported setup (HD 1920×960 + UHD 3840×1920 profiles, presets-only list, a loaded HD video) — 8/8 checks pass:

• UHD reopens as 3840×1920 (not the 3840×2160 preset);
• a loaded 1920×960 video does not override the UHD profile;
• the profile is still 3840×1920 after save-on-close (no corruption);
• a custom 1920×960 doesn't get shadowed by the 1920×1080 preset;
• in-session profile switching round-trips;
• standard presets and the video-as-fallback (when a profile has no resolution) still work.

Built the UI project clean. Not driven through the live GUI, but the affected logic is covered by the harness above.

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