feat(file-uploader): merge upload texts into single button label

[spliffone]

The uploadTextFileSelect input has been removed. The whole prompt is now rendered as the file-select button text via uploadDropText, avoiding fragile translation concatenation. A migration removes leftover usages.

BREAKING CHANGE: The uploadTextFileSelect input has been removed from SiFileUploaderComponent and SiFileDropzoneComponent.

The full text now lives in uploadDropText, which renders as the button. Remove uploadTextFileSelect and move its content into uploadDropText.

Before:

<si-file-uploader
  uploadDropText="Drop files here or"
  uploadTextFileSelect="click to upload"
/>

After:

<si-file-uploader uploadDropText="Drop files here or click to upload" />

---

Documentation.
Examples.
Dashboards Demo.
Playwright report.

Coverage Reports:

• charts-ng
• dashboards-ng
• element-ng
• element-translate-ng
• maps-ng
• native-charts-ng

[gemini-code-assist]

Code Review

This pull request simplifies the file uploader and dropzone components by removing the 'uploadTextFileSelect' input and combining its functionality into the 'uploadDropText' input. The default text is updated to "Drop files here or click to upload". To support this breaking change, a schematic migration ('migration-v51') has been added to automatically remove 'uploadTextFileSelect' from templates during the update to version 51. I have no additional feedback to provide as the changes are clean, well-tested, and include the necessary migrations.

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