Added visible border to iOS TextBox for better clarity in tutorials

[NikhilPatil2005]

General items:

• I have updated the relevant documentation files under docs/ (Not required for this UI-only iOS change)
• My code follows the:
  • Google Java style guide (for .java files)
  • Google JavaScript style guide (for .js files)
• ant tests passes on my machine (Not applicable for iOS Swift files)

If your code changes how something works on the device (i.e., it affects the companion):

• I branched from ucr
• My pull request has ucr as the base

Further, if you've changed the blocks language or another user-facing designer/blocks API (added a SimpleProperty, etc.):

• I have updated the corresponding version number in appinventor/components/src/.../common/YaVersion.java
• I have updated the corresponding upgrader in appinventor/appengine/src/.../client/youngandroid/YoungAndroidFormUpgrader.java (components only)
• I have updated the corresponding entries in appinventor/blocklyeditor/src/versioning.js

For all other changes:

• I branched from master
• My pull request has master as the base

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What does this PR accomplish?

Enhancement:
This pull request improves the visual clarity of the TextBox component on iOS devices by adding a visible border and light background color by default.

Currently, TextBoxes on iOS do not have any visible indicators unless a hint or background color is manually set, making it difficult for beginners to identify input fields.

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Description

• Added a light gray background color (systemGray6) to _field and _view in TextBox.swift.
• Added 1px border and slight corner radius to improve visibility.
• Changes affect only the iOS companion and do not alter Android behavior.

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[AppInventorWorkerBee]

Can one of the admins verify this patch?

[AppInventorWorkerBee]

Can one of the admins verify this patch?

[NikhilPatil2005]

check now