Keep project indicators fixed on the thread tree guide

[brsbl]

Human comments

What was wrong

Cross-project indicators followed truncated thread titles, so revealing row actions on hover moved the indicators horizontally.

What changed

• Project indicators sit on the left tree guide and stay fixed when actions appear.
• Nested titles keep their existing indentation, and sticky parent rows preserve guide continuity.
• Indicators retain their project tooltip and thread navigation.

How you verified

• Chrome for Testing 153.0.8010.36: desktop and mobile lists with nested, pinned, active, unread, and error threads.

• Hover keeps the indicator fixed; tooltip, selection, and action-menu behavior passed on the exact branch head.

• PR CI passed, including app tests and Linux/macOS package smoke checks.

Slop Cop review: pass with one follow-up

Reviewed head 3a1e7926b4bddf2a6ed07cc93fb39e82db51e5e5 against merge base e865697f56bea89f3413dd4cc7fae964850d20a0.
Scope: git diff e865697f56bea89f3413dd4cc7fae964850d20a0 3a1e7926b4bddf2a6ed07cc93fb39e82db51e5e5 -- apps/app/src/components/sidebar/ThreadRow.tsx.

Panel	Result
Security	No findings
Architecture	One P2, shared with correctness
Code quality	Same P2
Performance	No findings
End-to-end	Primary flow passed; P2 appearance confirmed

P2 follow-up: A cross-project icon can show a lighter square inside an unfocused split row.
The unconditional bg-sidebar utility at ThreadRow.tsx:475 overrides the shared split-state background.
Omitting that base utility for inactive split indicators would restore the row tint while preserving hover/menu states.
Verify leaf and sticky-parent indicators at rest, on hover, with actions open, and selected.
This non-blocking follow-up remains outside this PR under the review policy.

Before and after

Before: freshly fetched origin/main at 2c9fdf2dc02f09fb16003658f148bbd362cff0c2. After: 3a1e7926b4bddf2a6ed07cc93fb39e82db51e5e5.

Same isolated fixture, routes, interactions, light theme, scroll positions, and 2× density on both revisions.

Desktop · 1440 × 900

Thread list state	Before — origin/main	After — PR head
Idle / full list
Hovered active child
Selected parent
Actions open
Unfocused split

Mobile · 390 × 844

Thread list state	Before — origin/main	After — PR head
Idle / full list
Selected parent
Scrolled nested list
Actions open

BB-Thread-ID: thr_xqz2cxmmac

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