Multiple panel selection and bulk actions

**What the feature is (as Title)**
Multiple panel selection and bulk actions

**Value proposition**
Select several panels at once — shift-click or drag-to-select — then perform actions like moving, resizing, applying same colors, etc. in a single action. Restructuring a large dashboard today means repeating the same edit dozens of times; bulk actions make it one.

**Expected outcome**
Much faster dashboard maintenance and reorganization, more consistent layouts as alignment becomes a single operation rather than manual pixel-nudging, and reduced friction for the power users who own the biggest and most-viewed dashboards.

**Optional: Key user stories / use cases**

No files, tests, or entry points are mentioned. Start by clarifying the selection interactions, supported bulk actions, and dashboard scope with maintainers; done means users can select multiple panels and apply the agreed move, resize, color, and alignment operations consistently.

Written by the indexing model from the issue text.