feat: add QDashboardPanel component and useDashboardPanels composable

[ielvisd]

Adds resizable, collapsible, and localStorage-persisted dashboard panel
Extends QCard with drag-to-resize, collapse toggle, keyboard navigation, and a composable for managing multiple panels with shared state. Includes full tests, docs, and playground examples.

Hey Quasar team! 👋

This was created as an assignment to contribute something meaningful to a major open-source framework—I chose Quasar because I love it and have used it on previous projects. I

I have no expectation of a merge — I know big features like this need prior discussion and roadmap alignment. My main goal was the learning experience of building and contributing on a large codebase like Quasar.

That said, I followed the guidelines closely and aimed for production quality. Feel free to take anything useful or just close it. Thanks for building such an awesome framework! ❤️

What kind of change does this PR introduce?

• Bugfix
• Feature
• Documentation
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change?

• Yes
• No

The PR fulfills these requirements:

• It's submitted to the dev branch (or v[X] branch)
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix: #xxx[,#xxx], where "xxx" is the issue number)
• It's been tested on a Cordova (iOS, Android) app
• It's been tested on an Electron app
• Any necessary documentation has been added or updated in the docs or explained in the PR's description.

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to start a new feature discussion first and wait for approval before working on it)

Other information: