refactor(ui5-table): implement empty area rendering with custom focus outline

[aborjinik]

Summary :

• Renders a "dummy cell" (minmax(0, 1fr)) in tables where all columns have explicit widths, filling the remaining empty space to the right of the data columns
• Implements a custom per-cell focus outline using box-shadow that excludes the empty area, with full RTL support
• Moves shared row logic (focus(), _hasPopin, popin attribute management) from TableRow into TableRowBase so both data rows and header rows benefit
• Hides the row actions column when the table has no rows
• Reorganizes sticky cell and navigated cell CSS from TableRow.css into TableRowBase.css for proper inheritance
• Missing documentation review of feat(ui5-table-cell): merged property added #13297 is applied

Dummy cell rendering:

• Table.ts computes _hasFlexibleColumns and sets _renderDummyCell on all rows
• Grid template columns include a minmax(0, 1fr) dummy column positioned before or after actions depending on popin state
• Dummy cell has a left border (fixed column separator) when not in popin mode

Custom focus outline:

• When a row with a dummy cell is focused (non-popin), the standard outline is replaced with per-cell box-shadow insets
• _handleCustomFocusOutline() marks first/last visible cells via data-ui5-custom-outline attribute (start, end, or startend for single-column tables)
• CSS variables --_ui5_table_cell_custom_outline_block, inline-start, inline-end define the shadow values, with :dir(rtl) overrides

Refactoring:

• _hasPopin getter moved to TableRowBase with additional rows.length > 0 guard (prevents popin rendering in empty tables)
• Attribute renamed from _haspopin to _has-popin for consistency with kebab-case convention
• Navigated cell clip-path and focus rules consolidated using CSS nesting
• Sticky cell styles (position: sticky, z-index, background-color) moved to TableRowBase.css

[ui5-webcomponents-bot]

🚀 Deployed on https://pr-13462--ui5-webcomponents-preview.netlify.app