Implement ruler bounds visualization for the AABB of selected layers

[Keavon]

Partly closes #1969.

[gemini-code-assist]

Code Review

This pull request implements selection indicators on the document rulers, allowing users to see the extent of selected nodes directly on the ruler axes. The backend now calculates the selection's bounding box and transmits it to the frontend, which renders a line and endpoints on the RulerInput components. My feedback focuses on cleaning up a redundant CSS variable definition and correcting a CSS selector change that appears to break the layout of the top ruler.

[cubic-dev-ai]

2 issues found across 5 files

Confidence score: 4/5

• This PR is likely safe to merge, with only minor UI/CSS issues and no indication of functional breakage or data risk.
• Most severe issue: in frontend/src/components/panels/Document.svelte, .top-ruler .ruler-wrapper no longer matches RulerInput’s .ruler-input DOM, so top-ruler spacing styling may regress visually.
• In frontend/src/components/Editor.svelte, the duplicate color variable (#00a8ff vs rgb(0, 168, 255)) is dead code and mainly a maintainability cleanup rather than a runtime risk.
• Pay close attention to frontend/src/components/panels/Document.svelte, frontend/src/components/Editor.svelte - fix the selector mismatch causing ruler spacing regression and remove redundant CSS variable declarations.

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="frontend/src/components/panels/Document.svelte">

<violation number="1" location="frontend/src/components/panels/Document.svelte:895">
P2: The new `.top-ruler .ruler-wrapper` selector does not match `RulerInput`’s DOM (`.ruler-input`), so the top ruler spacing rule is no longer applied.</violation>
</file>

<file name="frontend/src/components/Editor.svelte">

<violation number="1" location="frontend/src/components/Editor.svelte:147">
P3: Duplicate CSS variable definition — `#00a8ff` and `rgb(0, 168, 255)` are the same color. The second declaration always overrides the first, making the first line dead code. Remove the redundant line.</violation>
</file>

_{Reply with feedback, questions, or to request a fix. Tag @cubic-dev-ai to re-run a review.}