Accessibility regression: User prompt visual distinction removed (#3390 fix) — needs opt-in restoration

### Describe the bug

The #3390 fix removed the background highlight from submitted user prompts in scrollback. This resolved a cosmetic complaint from a few users but silently removed an accessibility feature that aids cognitive parsing of long agent conversations.

### The case for accessibility

Terminal-based agent conversations produce a fundamentally different reading pattern than regular UI apps. A single user prompt can generate hundreds of lines of agent output — tool calls, code blocks, reasoning, results — before the next user prompt appears. This creates an extreme signal-to-noise problem which all terminal users love resolving with customizations to their prompts.

Without background distinction:

❯ fix the auth bug ← where is this?
● Exploring codebase (shell)
│ grep -r "authenticate" src/
└ 47 lines...
◐ Found the issue in src/auth/handler.ts...
● Editing src/auth/handler.ts
...
(200 more lines of agent output)
...
❯ now run the tests ← good luck finding this
● Running tests (shell)
│ npm test
└ 94 lines...

With background distinction:
Every ❯ line has a subtle background — your eye can scan a 500-line scrollback in seconds and locate every point where you spoke. This is the same principle behind every email client & messaging app ever — sent vs received visual distinction.

This isn't decoration. It's a well-established UX pattern for conversational interfaces that directly impacts:

1. Scan efficiency — Finding your last instruction in 500+ lines of agent output without visual anchors requires linear search. With background highlights, it's a single order visual scan.
2. Context recovery — After stepping away from a session, the first thing you do is scan for "what did I ask?" Background distinction makes this instant.
3. Error attribution — When debugging agent behavior, you need to quickly correlate "what I asked" → "what it did." Visual separation of user turns is essential.
4. Accessibility (WCAG) — Users with attention or cognitive processing differences benefit disproportionately from visual chunking of conversational turns. This is documented in WCAG 1.3.1 (Info and Relationships) (https://www.w3.org/WAI/WCAG21/Understanding/info-and-relationships.html) — meaningful structure should be programmatically or visually conveyed.

The original complaint was cosmetic; the loss is functional
Issue #3390 described the background as "unpleasant" and "low-contrast." That's a theming problem — the fix should have been adjusting the shade or respecting terminal background color, not removing the feature entirely. The correct response to "this color is ugly" is to fix the color, not to remove the distinction.

### Prior art — the community has been asking for this

Multiple open issues request exactly what was removed:
#2746 │ Visual distinction between user input and assistant responses
#2288 │ Visual differentiation for user input text
#1862 │ Highlight user input in scroll history
#2123 │ Fine-grained per-element theming (prompt vs response foreground)

Four separate requests for this feature, and the one version that shipped it got reverted because of a color choice.

### Affected version
Background removed: ≥ 1.0.53, tested up to 1.0.56

### Steps to reproduce the behavior
Upgrade to the latest versions

### Proposal

1. Restore the background highlight on submitted user prompts as the default behavior
2. Make it theme-aware — pick a shade that works against both light and dark terminal backgrounds (the actual #3390 complaint)
3. Add a config toggle for users who truly prefer no distinction:
{ "userPromptHighlight": false }
4. If full background is controversial, alternatives that still provide visual anchoring: - A colored left border (like GitHub PR comments)
- A subtle │ gutter marker in a distinct color
- Bold/colored ❯ chevron (currently already exists but insufficient at scale)

### Additional context

Tested on Ubuntu and MacOS, light and dark background terminal

Start by reviewing the behavior change from #3390 and the reported versions 1.0.53–1.0.56, then compare current prompt rendering with the examples in this issue. Done means user prompts regain a visible, theme-aware distinction and users can disable it with the proposed configuration, with the result checked on light and dark terminal backgrounds.

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