fix(agents): bin path override + envOverride + Windows shell launch

[joeylee12629-star]

Closes #13
Closes #15

Summary

Two reported issues, one PR — both live in the agent-invocation layer and the fixes touch the same files (detect.ts, invoke.ts).

Issue	Reporter	Symptom	Fix
#15	@LuckyFishGeek	Windows EINVAL ("无效的参数") on every Convert / Draft	shell: true on win32 only, so npm-installed .cmd agent shims can launch
#13	@hallestar	Auto-detection picks the wrong binary; no UI to override; Scoop on Windows not covered	Custom-path field in Settings + invocation layer honors override / env / PATH; userToolchainDirs() adds Scoop layouts

Two commits, ordered surface-down → engine-up so review reads naturally:

1. feat(settings): UI for custom agent binary path + Scoop autodetect (#13) — Settings UI, store schema (v5 → v6), client-side wire-through, Scoop directory coverage.
2. fix(agents): respect bin override + envOverride; use shell on win32 (#13, #15) — invocation-layer bin resolver + the win32 spawn fix.

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#15 — Windows spawn EINVAL

Root cause — npm installs every supported CLI agent (Claude Code, Cursor Agent, Gemini CLI, ...) as a .cmd shim on Windows. Node's child_process.spawn cannot launch a .cmd without going through the shell. macOS / Linux exec the binary directly so the same code path worked there.

Fix — shell: process.platform === "win32" in the spawn options. Unix is unchanged.

Safety — the prompt travels via stdin (stdin protocol) or --message <text> (argv-message protocol), never interpolated into a shell command line. The --model value comes from a curated list, not free-form user input. So flipping shell: true does not open up shell-injection.

Diagnosed and proposed by @LuckyFishGeek in #15.

#13 — Manual binary path override + Scoop on Windows

Root cause analysis (from @hallestar in the issue):

• detect.ts defines envOverride per agent (e.g. CLAUDE_BIN) but invokeAgent() ignored it — it re-resolved via resolveOnPath() only.
• The Settings UI had no input for a custom path. Once auto-detection picked the wrong binary, the user was stuck.
• userToolchainDirs() did not cover Scoop-managed Node, where global npm shims land in C:\Users\<user>\scoop\apps\nodejs\current\ (no /bin/ subdirectory).

Three pieces in this PR:

1. Bin resolver in invoke.ts. New resolveBinForAgent() returns one of { ok | override-missing | not-found }. Tries, in order:

   • opts.binOverride (Settings UI). If set but does not exist, hard-fail with a clear error instead of silently falling through. Users who typed a path deserve to see the typo, not run a different binary by accident.
   • process.env[def.envOverride] (e.g. CLAUDE_BIN).
   • PATH scan over def.bin then def.fallbackBins.

2. Settings UI ("Custom path" panel). Each installed agent card now has a custom-path panel below the model picker. Empty = use auto-detection. Filled with an absolute path = override. Save / Clear buttons; Enter / Esc shortcuts; the auto-detected path is shown next to the input as a reference.

3. Scoop coverage in userToolchainDirs(). New scan paths for Windows:

   • %SCOOP%\shims, %SCOOP%\apps\nodejs\current, %SCOOP%\apps\nodejs-lts\current
   • %SCOOP_GLOBAL%\shims, %SCOOP_GLOBAL%\apps\nodejs\current (default C:\ProgramData\scoop)
   • %APPDATA%\npm (default standalone-installer location)
   • Existing NPM_CONFIG_PREFIX now also scans the prefix dir directly, not just <prefix>/bin — npm on Windows installs shims one level up.

Persistence. agentBinOverrides: Record<string, string> lives on the persisted store with a v5 → v6 migration that initializes it to {} on existing browsers. Empty values are pruned by the setter.

Wire-through. useConvert and useDraft read the override for the current agent and ship it as binOverride on the /api/convert and /api/draft request bodies.

i18n. Six new agent.customBin.* keys (en + zh-CN).

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Verified

End-to-end with Playwright + a real browser session:

• Settings → Claude Code → "Custom path" panel renders with eyebrow / subtitle / auto-detected path / placeholder hint / Save / Clear.
• Type /no/such/path/claude → Save → localStorage.html-everything-store → state.agentBinOverrides = { claude: "/no/such/path/claude" }.
• Convert / Draft with that override → event: error with "Claude Code: custom path \/no/such/path/claude` does not exist. Update or clear it in Settings → Custom path."`
• Clear button → store clears → next Convert / Draft falls back to PATH scan and works normally.
• pnpm exec tsc --noEmit clean; pnpm build clean.

Win32 shell-launch fix is by inspection — does not regress macOS (PR was developed on macOS arm64 and continues to work). @LuckyFishGeek's repro instructions match the fix; once this lands they can re-test on Windows 11 + Scoop.

Test plan for reviewers

• On Windows 11, with Claude Code installed via Scoop / npm-on-Scoop, Convert and Draft both work without EINVAL.
• Settings → any installed agent → Custom path panel renders. The auto-detected path is shown.
• Filling in an absolute path that exists → Save → Convert uses that binary (visible in the Run log's spawn … line).
• Filling in an absolute path that does not exist → Convert immediately surfaces the actionable error.
• Clear → falls back to auto-detection.
• Existing users (v5 store) → no migration error; agentBinOverrides initialized to {}.

Out of scope

• Multi-binary fallback chain UI ("try this path, then this path, then PATH"). Single override field is enough for the reported failure modes; can be extended later if needed.
• Auto-test of the binary on save (run <bin> --version and warn if it errors). Same — can layer on later; the actionable error on Convert is already enough signal.
• Generalizing HTML_ANYTHING_AGENT_PROXY / extra-agent env hooks (this is what PR fix: harden agent conversion and localize fonts #14 covers, separately).

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