fix: adjust file picker for background image to allow folder navigation

[cristianscheid]

• Resolves: [Bug]: Can't put custom wallpaper / background from personal space #58645

Summary

Currently, the file picker to change user background (under settings/user/theming) does not allow to navigate folders when searching for a file. Instead, it only shows files from root directory on "All files" tab.

This happens because of setFilter((node) => node.mime.startsWith('image/')), which causes folders to be filtered out from results. This PR changes this line to use setMimeTypeFilter(['image/*']), which allows to navigate folders while mantaining the filter to only allow image selection.

Screenshots

Before	After

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

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

[susnux]

If possible it would be really great to have a small PlayWright test for this

[cristianscheid]

@susnux @pringelmann small test added. This is the first Playwright test I wrote, would appreciate some feedback.

[cristianscheid]

/compile amend/

[cristianscheid]

@susnux should we backport this down to stable32?

[susnux]

/backport to stable34

[susnux]

/backport to stable33

[susnux]

/backport to stable32

[backportbot]

The backport to stable32 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable32
git pull origin stable32

# Create the new backport branch
git checkout -b backport/61688/stable32

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 15c336f4 1f1928df

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/61688/stable32

Error: Failed to check for changes with origin/stable32: No changes found in backport branch

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Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

[cristianscheid]

The backport to stable32 failed. Please do this backport manually.

backport to stable32 is not needed, as setMimeTypeFilter() was already being used:
https://github.com/nextcloud/server/blob/stable32/apps/theming/src/components/BackgroundSettings.vue#L242