feat(v0.2): PromptV2 + chat redesign + theme variants + headless browser

[SimpleLittleDev]

Summary

Stacked on top of #3. Continues the v0.2 rollout with Phase 2 (input + chat redesign), Phase 5b (headless browser engine), and Phase 6 (theme variants). All 146 tests pass; lint, typecheck, and build are clean.

Phase 2 — Input editor + chat redesign

• src/lib/inputHistory.ts — persistent JSONL history at ~/.fastcode/history. Skips empty / exact-duplicate entries, recovers from corrupt lines, trims to a configurable max, falls back to in-memory when the disk is read-only (Termux).
• src/tui/PromptV2.tsx — multiline editor written directly on useInput (no ink-text-input shim). Features:
  • Shift+Enter / Ctrl+J / Meta+Enter insert a newline; bare Enter submits.
  • ↑ / ↓ recall history when on the first / last line (preserves draft on Down-back-to-edge).
  • Tab autocompletes the slash command on the first token; otherwise inserts a 2-space soft tab.
  • Ctrl+U kills the current line, Ctrl+W deletes the previous word, Ctrl+A / Ctrl+E jump to line start / end, Ctrl+K kills to end of line.
  • Detects bracketed-paste bursts (multi-char input chunks with no modifiers) and inserts the chunk verbatim, including embedded newlines.
  • Cursor block highlights via <Text inverse>; blink toggles unless stable (or noanim mode) is set.
• src/tui/DiffView.tsx — small unified-diff parser (parseDiff) + themed render. Truncates with a …N more lines footer past maxLines.
• src/ui/Message.tsx — BubbleRole extended with tool, error, warn, success, command, diff. Each variant has a distinct sigil + color routing through the new theme fields. The diff role embeds <DiffView> directly. Bold suppressed when theme.noColor is set.
• src/app.tsx — Prompt → PromptV2 with historyRef; submitted messages persist before clearing the input. Tool calls now render as tool (call) followed by success / error / diff (result, diff auto-detected). Cancellation uses the warn role.

Phase 5b — Headless browser engine

• src/browser/headless.ts — createHeadlessEngine() lazily imports playwright-core (kept as an opt-in peer dep, not added to dependencies). Uses chromium.launch({ headless: true }), fetches with domcontentloaded wait, extracts document.body.innerText (serialized as a string so we don't need DOM types in our tsconfig), and clips to maxChars. Throws HeadlessEngineUnavailableError with a friendly install hint when the import fails.
• src/browser/index.ts — new loadBrowserEngineDetailed({ prefer, strict, policy }) returns { engine, kind }. prefer=auto upgrades to headless when available, prefer=headless non-strict silently falls back to text, prefer=headless strict throws. The legacy loadBrowserEngine() delegates to the detailed variant, so existing callers are unchanged.

Phase 6 — Theme variants

• src/config/themes.ts — adds cyber (neon magenta/cyan), nocolor (single-color, sets noColor=true so bold is suppressed), hicontrast (a11y, yellowBright + whiteBright on bright backgrounds), and termux (avoids *Bright variants that look bad on AMOLED).
• pickThemeForCaps({ forceNoColor, os, colorDepth }) automatically picks nocolor for dumb terminals and termux on Android.
• src/types.ts — Theme gains optional warn, tool, command, diffAdd, diffDel, noColor. All five existing themes still validate (the new fields are optional and fall through to existing error / accent / success).

Tests added (25 new, 146 total)

• test/inputHistory.test.ts (5) — file creation, recall, dedupe, corrupt-line tolerance, trim.
• test/promptHelpers.test.ts (5) — clamp, posFromOffset, autocompleteCommand.
• test/diffView.test.ts (3) — kind classification, header vs add/del precedence, blank lines.
• test/themes.test.ts (6) — variant presence, noColor palette, getTheme fallback, pickThemeForCaps matrix.
• test/headlessEngine.test.ts (5) — isHeadlessAvailable returns false without playwright-core, prefer=auto and prefer=headless-non-strict fall back to text, prefer=headless-strict throws HeadlessEngineUnavailableError.

npm run check (test + typecheck + lint + build) passes locally.

Review & Testing Checklist for Human

• PromptV2 paste flow: paste a multi-line JSON / log into the prompt and confirm newlines aren't swallowed and the bubble renders alignment-correct (regression-prone area: bracketed paste vs single-keystroke ambiguity).
• History recall: send 3 messages, press ↑ three times to walk back, ↓ to walk forward to the empty draft. Confirm ~/.fastcode/history contains JSONL entries.
• Theme switch: /theme nocolor, /theme termux, /theme cyber, /theme hicontrast and verify each remains readable. Especially check nocolor on a real NO_COLOR=1 terminal.
• Tool bubbles: trigger any agent tool call (shell, file_read, etc.) and check the new tool → success / error blocks render with distinct sigils. If the tool emits a unified diff in stdout, confirm the diff block colorizes +/- lines.
• Headless fallback: without installing playwright-core, confirm the app still boots and loadBrowserEngine({ prefer: "headless" }) returns a working text engine. Then npm i playwright-core && npx playwright install chromium and confirm prefer=headless actually launches Chromium.
• Cross-OS smoke: at minimum boot the TUI on Linux (CI) and on Windows / WSL or Termux locally; check that the prompt cursor block doesn't desync after resize.

Notes

• Stacked on top of feat(v0.2): living TUI + MCP + sandbox v2 + browser tool #3 — base is main, so this PR's diff includes both phase 1 and phase 2/5b/6 commits. Easier to land as one if feat(v0.2): living TUI + MCP + sandbox v2 + browser tool #3 is approved; we can also rebase to phase-1's branch if you want them split.
• playwright-core is intentionally not added to dependencies or peerDependencies in package.json. Keeping it user-installed avoids forcing a 50 MB transitive dep on Termux / VPS users who don't want a browser. The error message in HeadlessEngineUnavailableError tells them how to opt in.
• Old src/ui/Prompt.tsx is untouched (still on disk) for backwards compat in case someone external imported it. Safe to delete in a follow-up.

Link to Devin session: https://app.devin.ai/sessions/0e73b43883c04a17b651ca6aa7dc1a5e
Requested by: @SimpleLittleDev

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