feat: improve horizontal overflow cues and off-screen task attention

[ASRagab]

Summary

This PR improves the usability of Parallel Code when many task slices are open side-by-side.

The key change in framing was that horizontal scrolling already existed; the real product gaps were:

1. overflow discoverability — it was easy to miss that more slices existed off-screen
2. off-screen observability — hidden agent/task slices could need attention without a strong signal in the sidebar

This PR addresses those gaps and also increases terminal scrollback for agent review/debugging.

What changed

1) Horizontal overflow affordance

• adds passive left/right edge cues to the main tiling strip in src/components/TilingLayout.tsx
• tuned the visual treatment in src/styles.css to use a narrow accent/caret-led cue rather than a heavy overlay
• preserves the existing horizontal scrolling model instead of introducing custom scroll controls or a layout rewrite

2) Runtime-only viewport visibility for task slices

• adds runtime-only task visibility state:
  • visible
  • offscreen-left
  • offscreen-right
• publishes that state from the actual horizontal viewport in TilingLayout
• intentionally does not persist viewport visibility

3) Additive task attention model

• adds a separate attention model in src/store/taskStatus.ts without replacing the existing dot status model:
  • idle
  • active
  • needs_input
  • error
  • ready
• keeps getTaskDotStatus() intact for backward compatibility

4) Better off-screen/background attention detection

• extends throttled output analysis so background task agents can still surface meaningful attention state
• fixes an important edge case where throttled background [Y/n] prompts could otherwise lose needs_input state

5) UI surfacing for off-screen attention

• updates Sidebar and StatusDot so off-screen tasks can surface stronger attention cues
• keeps the UI relatively quiet:
  • active off-screen tasks get visual emphasis
  • explicit text badges are reserved for stronger states like input/error
• updates edge affordances so they can reflect off-screen attention directionally

6) Attention-aware desktop notifications

• moves notification logic from coarse busy/waiting transitions toward richer attention transitions:
  • ready
  • needs_input
  • error
• keeps debounce/deduping behavior

7) Longer terminal scrollback

• increases xterm scrollback to 10_000 lines in both:
  • src/components/TerminalView.tsx
  • src/remote/AgentDetail.tsx
• centralizes the shared value in src/lib/terminalConstants.ts

Why this approach

• does not rewrite the layout system
• does not rewrite focus/navigation behavior
• keeps viewport state runtime-only
• prefers additive observability over broad architectural change
• treats task-agent attention as the first pass, rather than trying to model every shell-specific state immediately

Notes on implementation details

• viewport measurement uses actual DOM rects against the scroll container rather than brittle offset math
• recompute triggers were tightened so reorder/structure changes do not leave stale off-screen state behind
• the overflow cue went through a few iterations to land on a narrower, more natural accent/caret-led affordance that works better across themes

Validation

• npm run typecheck
• npm run lint
• npx vitest run src/store/taskStatus.test.ts

All passed locally.

Files changed

• src/components/Sidebar.tsx
• src/components/StatusDot.tsx
• src/components/TerminalView.tsx
• src/components/TilingLayout.tsx
• src/lib/terminalConstants.ts
• src/remote/AgentDetail.tsx
• src/store/core.ts
• src/store/desktopNotifications.ts
• src/store/store.ts
• src/store/taskStatus.test.ts
• src/store/taskStatus.ts
• src/store/types.ts
• src/store/ui.ts
• src/styles.css

Commit structure

• feat: add horizontal overflow affordance cues
• feat: surface off-screen task attention
• feat: increase terminal scrollback
• chore: simplify sidebar attention cleanup