feat: add visual blueprint editor for custom blueprints

[EpicLPer]

DISCLAIMER: AI generated code with Cursor. See Discussion here about this: #38

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Summary

Draft PR for a visual blueprint editor — feedback welcome before merge.

Adds a self-contained editor view for user/custom blueprints (bundled blueprints remain read-only):

• Live SVG preview with drag-to-move and resize handles (rect/circle)
• Custom SVG path (d) button support
• Create + edit custom blueprints with PNG upload (auto-resized to max 800×500)
• Undo/redo and unsaved-changes warnings (cancel, browser back, refresh/close)
• Backend blueprint/save websocket with YAML validation; blocks overwriting bundled blueprints

Architecture (v4 frontend)

• New view: frontend/src/switch-manager-blueprint-editor.ts (standalone Lit component)
• Routed from switch-manager-panel.ts for /switch_manager/blueprint/new and .../edit/:id only
• Does not extend the shared HA_COMPONENTS runtime load list
• Entry points: “Create Blueprint” + edit pencil in blueprint selector (custom blueprints only)
• Bundle rebuilt via cd frontend && npm run build

Backend

• blueprint/save websocket endpoint
• Save helpers with YAML validation, PNG limits, editable guard on bundled blueprints
• Panel reload fix (async_remove_panel on setup/unload)

Maintenance note

I may not be able to maintain this long-term; happy for ongoing upkeep to live with the project if it lands.

Test plan

• Create new custom blueprint: upload image, place buttons, save
• Edit existing custom blueprint; confirm bundled blueprints cannot be edited
• Drag/resize buttons; undo/redo
• Cancel / browser back with unsaved changes shows confirmation
• Integration reload does not error on panel registration
• cd frontend && npm run build produces the committed bundle