Simplify the file conflict dialog

[jancborchardt]

Same as nextcloud-libraries/nextcloud-dialogs#2540

Our current file conflict dialog (when you upload the same file again) immediately jumps into a very complex "pick files" view.

Based on a mockup by @kra-mo and some discussions.

The improvements:

• "Existing version" has been moved to the left and "New version" to the right. This is swapped from the previous layout as that was confusing – new things are usually on the right.
• The flow for 1 file was simplified by removing the checkboxes altogether, and just going for buttons "Keep both" and "Replace".
• For multiple files, the newer files are all preselected by default. That way people can click "Continue" directly.
• The wording is simplified
• To more easily compare what’s new or what changed between existing and new, whichever date is newer is bolded, and whichever size is larger is bolded.
• A little arrow inbetween each of the versions pointing right from the existing file to the new file helps with visual clarity.
• The icons of the cancel and skip buttons were removed to reduce visual noise.

We could probably simplify the wording even more for the multiple files case, but this PR is also quite big already. :)

Before	After

Mockup by @kra-mo for reference

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI

[codecov]

Codecov Report

❌ Patch coverage is 0% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 35.47%. Comparing base (fa56e3c) to head (d288028).
⚠️ Report is 26 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/dialogs/components/ConflictPicker.vue	0.00%	13 Missing ⚠️
lib/dialogs/components/NodesPicker.vue	0.00%	9 Missing ⚠️
lib/dialogs/components/NodePickerCard.vue	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2151      +/-   ##
==========================================
- Coverage   36.30%   35.47%   -0.83%     
==========================================
  Files          17       18       +1     
  Lines         774      792      +18     
  Branches      145      150       +5     
==========================================
  Hits          281      281              
- Misses        480      498      +18     
  Partials       13       13              

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