refactor: prune dead editor sass and consolidate duplicates - #2463

What's Changed

Prunes dead SASS and consolidates exact duplicates across the editor, code editor, launch and error stylesheets, with no intended visual change.

• Removes ~3,500 rules that cannot match anything the editor creates: legacy pre-PCUI ui-* attribute rules, rules for removed panels, dead state classes, and selectors that Sass @extend wove into impossible combinations (e.g. .font-icon woven into ~400 selectors)
• error.scss no longer imports the whole PCUI theme; it keeps the few values and the scrollbar rules the static error pages use
• The code editor imports a new pcui/_pcui-core.scss (theme and tooltip only) instead of every editor-only PCUI extension
• Drops declarations that are always overridden or that restate PCUI's own identical rules (color, gradient and curve inputs, color picker, font utilities)
• Consolidates exact duplicates into shared mixins and placeholders: font placeholders, ghost-button states, fullscreen glyph, VC frames/lists, alert bar, strike-through; editor-wide tokens and mixins now live in _editor-tokens.scss
• Merges identical keyframes into spin-360 and skeleton-pulse
• Moves the MCP account popover styles into _editor-mcp.scss and folds _editor-scene-size-alert.scss into _editor-toolbar-alert.scss; deletes the empty _editor-settings-ai.scss

Verification: every change was checked with a cascade-aware comparison of the compiled CSS against main, and by swapping the old and new stylesheet into the running editor and diffing the computed style of every element (plus ::before/::after) and a screenshot. That covered the projects page, the idle editor, every entity and asset inspector in a sample project, scene settings, the color, version control, builds, project and team pickers, the code editor with a script open, the launch page and an error page. All showed zero differing styles.

Rules that never matched but look like they were meant to (removed as-is, worth a follow-up fix):

• .script-asset-inspector-attribute-warning.clickable-warning: clickable script warnings never got the pointer/underline
• .pcui-menu-item-postfix.disabled: PCUI uses pcui-disabled, so the disabled paste label was never hidden
• .cms-card-desc.empty, .dev-log-post-actions { gap: 2px }: same value as, or overridden by, the parent rule

Manual smoke test:

1. Open the projects page. Cards, list rows, the sidebar, the account menu and the MCP popover look unchanged.
2. Open a project and select entities with different components. Inspector panels, headers and fields look unchanged.
3. Select a material, texture, script, font and template asset. Each inspector and preview looks unchanged.
4. Open scene settings, then a color field's color picker. Both look unchanged.
5. Open Version Control: the branch list, history, diff and the progress widget look unchanged (no border on the progress status card).
6. Open Builds & Publish. Apply a filter and open its menu: the button stays dark while the menu is open.
7. Open the code editor with a script. The file tree, tabs and Monaco look unchanged.
8. Launch a scene. The launch page chrome looks unchanged.

Checks

• I confirm I have read the contributing guidelines