[FEATURE] Desktop: carry project/session colors into split-view panes

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

In split view I run 2–4 sessions side by side, often from different projects. Every pane looks the same, so I lose track of which pane is which, and I've typed instructions into the wrong session more than once.

#96710 asks for sidebar colors, which helps you find a session. Split view is where the mix-up actually costs you, because the wrong agent gets the instruction.

Proposed Solution

Let people color projects, groups and sessions, and carry that color from the sidebar into the panes.

• Pick a color: right-click a project, group or session → Color (swatches, or Inherit).
• Inheritance: a session uses its own color, then its group's, then its project's.
• Panes: the focused pane gets a bold outline in its color, and unfocused panes get a faint one. The header's project tag keeps the project's color, so a session colored by its group still shows its project.
• Sidebar: the focused session's row and section get a light fill, other open sessions a fainter one, and each row gets a thin color bar.
• One setting for how much color: Off (today's app), or any mix of Accent (top bar), Border (outline plus tinted composer) and Tint (light wash).

Alternative Solutions

• Pane titles are already there, but they're small and look alike at a glance.
• Sidebar-only colors (#96710) help you find sessions, not tell open panes apart.
• Custom themes are app-wide, not per project or session.

Priority

Medium - Would be very helpful

Feature Category

Other

Use Case Example

1. I have three panes open: a feature in project A (violet), a bug in project B (cyan), docs in project C (green).
2. I click into B. It gets a bold cyan outline, and its sidebar row fills cyan.
3. I can tell the panes apart without reading titles, so I don't type into the wrong one.

Additional Context

Mockups below: a 29-second video, five stills, and a clickable HTML mockup in the zip (open it in a browser: click panes to focus them, ⌘-click sessions to split, right-click anything to recolor it). All data is fake.

Related: #96710 (sidebar colors by project), #38036 (sidebar grouping).