feat(ui): inline short pastes, only collapse when >= 5 lines

[KillerQueen-Z]

Problem

Every bracketed-paste — even a one-line prompt — was being replaced in the input box with a [Pasted ~1 line] placeholder. Concretely, pasting:

"Buy a US phone number, then call +1-555-0123 in English and ask whether they're free for a 30-minute product demo on Thursday. Use a warm female voice and don't talk for more than two minutes."

(1 line, ~230 chars) collapses to [Pasted ~1 line]. The user can't see what they pasted, and the LLM only sees the decoded content at submit time — but the in-flight UX is broken for short snippets.

Root cause: findPasteBlocks(...) > 0 triggered the collapse unconditionally — no threshold check at all.

Fix

Mirror Hermes's behavior: only collapse pastes of >= 5 lines into a [Pasted ~N lines] block. Shorter pastes get inserted as plain text so they render verbatim and behave like normal typed input.

The threshold lives in a single named constant (PASTE_COLLAPSE_LINE_THRESHOLD) right next to the other paste markers, so it's easy to promote to a config field later if needed.

Comparison with other agents

Agent	Threshold
Claude Code	>= 4 lines
opencode	>= 3 lines OR > 150 chars (community has been complaining this is too aggressive)
Hermes	>= 5 lines (adopted here)
Cursor	~10 lines / ~1 KB
Aider	never collapses

Behavior table

Paste	Before	After
1-line, 230-char prompt	[Pasted ~1 line] ❌	inline ✅
2-line shell command	[Pasted ~2 lines]	inline ✅
4-line stack trace	[Pasted ~4 lines]	inline ✅
5-line code block	[Pasted ~5 lines]	[Pasted ~5 lines] ✅
50-line log dump	[Pasted ~50 lines]	[Pasted ~50 lines] ✅

Test plan

• npm run build passes
• Manual: paste the demo prompt above — renders inline, no [Pasted] placeholder
• Manual: paste a 5+ line snippet — collapses to [Pasted ~N lines] as before
• Manual: navigate with arrow keys / backspace through both short and collapsed pastes — no regressions
• Submit decodes correctly in both branches (collapsed pastes still expand to original content at submit time)

Scope

Single-file change to src/ui/app.tsx. No new dependencies. No behavior change for pastes that exceed the threshold.