feat(app): sidebar reveal animation, hover peek overlay, and weaker dividers

[iamdavidhill]

Issue for this PR

Closes #

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

The sidebar now feels like it is always present underneath the main content instead of popping in, and the layout no longer pushes session content off-screen when the sidebar opens.

• Desktop: the main area slides and shrinks to reveal the sidebar (session + filetree keep their widths; the review panel flexes to absorb the space change).
• Desktop: hovering a project avatar brings back the peek sidebar overlay, animated as a slide-in over the main area, with a right-only shadow clipped to the main region.
• UI polish: introduced border-weaker-base for subtle dividers and applied it to the session side panel dividers, terminal tabs divider, and the review tabs list divider.
• Mobile web: sidebar uses the full device width with a max-width of 400px.

These changes work by keeping the sidebar rendered behind the main layer and animating the main layer’s left edge, so the viewport width stays stable while the review column takes the hit when space is tight.

How did you verify your code works?

• bun run typecheck in packages/app
• bun run typecheck in packages/ui

Screenshots / recordings

TODO: add a short recording showing open/close + hover peek overlay.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR