feat: add stack-on-phone attribute to os-table - #981

Thank you for taking on #900, @Pranjal1423! Keying the layout off the shell's mode stamp instead of a media query is the right call, and the help block and components reference are nicely updated.

One case still slips through, and it's the one #900 is about. In an .os.php view the app runtime's morph (syncAttributes() in src/app-runtime/morph.ts) removes every attribute the server didn't paint and puts back the ones it did. So on a phone, the first action drops stacked and restores sticky-columns, and the table turns back into a grid. The table only re-checks when data-os-mode changes. Since stacked and stickyColumns are already observed props, re-syncing on those in attributeChangedCallback() should be enough (stackOnPhone() only writes when something differs, so it settles after one pass):

// os-table.ts, attributeChangedCallback()
if (
	( name === 'stacked' || name === 'sticky-columns' ) &&
	this.isConnected &&
	this.hasAttribute( 'stack-on-phone' )
) {
	this._syncStackOnPhone();
}

A test that runs morphChildren over a stacked table in phone mode would pin it. Also, the "How to Test" steps say data-os-mode="phone", but the stamp value is mobile.

Thank you for the thorough review, @epeicher!

That makes complete sense — on .os.php view morphs, syncAttributes() dropped stacked and restored sticky-columns, reverting the table back to a grid without triggering the mode stamp observer.

In commit 984703b, I have:

1. Re-synced on observed attributes: Added observation and re-syncing for stacked and sticky-columns in attributeChangedCallback(), guarded against re-entrancy so it settles in a single pass.
2. Preserved sticky columns across morphs: Retained the original sticky-columns count on the element instance so that when syncAttributes() cleans up unpainted attributes (like data-os-sticky-columns), returning to desktop mode or dynamically removing stack-on-phone cleanly restores the original pinned columns.
3. Added regression tests: Added unit tests in os-table-stack-on-phone.test.ts running morphChildren() over <os-table stack-on-phone> in mobile mode, verifying both morph persistence and subsequent desktop/grid restoration.
4. Updated PR description: Corrected the "How to Test" section in the PR description to use data-os-mode="mobile" instead of phone.