fix(web): harden git checkout, run lifecycle, and branch-picker UX

[cnjack]

Follow-up fixes for issues found while reviewing the (already-merged) #96. Each was reproduced against the merged code and verified; the proposed fixes from review were corrected where wrong (notably: a -- separator does not fix the checkout injection, and context.CancelFunc can't be compared — a generation counter is used instead).

Backend changes are additive or behavior-narrowing; frontend changes are guard/copy fixes. No API breakage.

🔴 HIGH

• git checkout argv injection → silent data loss. branch flowed straight into the git argv, so {branch:"-f"} ran git checkout -f, discarding all uncommitted work and returning 200 OK. Now reject branch names beginning with - (valid git refs never do). internal/web/git.go
• Lost-cancel race → /stop silently dead. A finishing run's defer could nil the next run's runCancel (gated only by the running CAS), leaving running=true, runCancel=nil — /stop returned stopped but cancelled nothing. Now guarded by a per-run generation (Engine.runGen): a superseded defer is a no-op. internal/web/engine.go, internal/web/server.go

🟠 MEDIUM

• Stash orphaned on failed switch. A stash-strategy checkout that fails after auto-stashing now pops the stash to restore the tree (and names the stash if pop fails) instead of leaving work in stash@{0} with no hint. git.go
• Blocked-checkout classification. New kind (tracked|untracked) field on the blocked response so the UI can pick safe recovery options. git.go
• setActiveEngine reclaim TOCTOU. The running check is folded into the emu-guarded block so a concurrently-starting run on the outgoing engine isn't torn down. engine.go
• Deleting the open conversation. Resets the chat view (clearChat + blank currentSessionId) only when the deleted task is the active one, so the next message can't be sent to a dead session; background-task deletes are unaffected. Sidebar.vue

🟡 LOW

• handleSessionDiff captures the active engine once so snapshot and pwd come from the same repo across a swap. server.go
• handleValidatePaths reports a path missing only on confirmed not-exist, not on transient/permission stat errors that were hiding valid workspaces. server.go
• Branch-switch copy no longer claims only "changes" and warns that discard deletes untracked files (en/zh-Hans/zh-Hant/ja/ko).
• BranchPicker Cancel routes through reset() so a stale error no longer leaks into the branch list. BranchPicker.vue
• ProviderIcon initial-fallback regex gains the g flag. ProviderIcon.vue

Tests

Adds internal/web/git_test.go: checkout dash-reject (asserts uncommitted work survives), blockKind tracked/untracked, and validate-paths missing detection (incl. the EACCES case the fix targets).

Deferred (own follow-up)

The kind-consuming UI (hide discard for untracked), the #9 backend porcelain classification, and a post-switch "stashed" toast — larger UX surface, out of scope for this bug-fix batch. The backend kind field ships now so that follow-up needs no API change.

Verification

go build ./... · go vet ./internal/web/... · go test -race ./internal/web/... · npm run build (type-check + vite) · oxlint + eslint on changed files — all green.

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: feb3ba47-e0b4-4a0a-9275-0768257f4b42

📥 Commits

Reviewing files that changed from the base of the PR and between e697fb2 and e22e4c6.

📒 Files selected for processing (12)

• internal/web/engine.go
• internal/web/git.go
• internal/web/git_test.go
• internal/web/server.go
• web/src/components/BranchPicker.vue
• web/src/components/ProviderIcon.vue
• web/src/components/Sidebar.vue
• web/src/i18n/locales/en.ts
• web/src/i18n/locales/ja.ts
• web/src/i18n/locales/ko.ts
• web/src/i18n/locales/zh-Hans.ts
• web/src/i18n/locales/zh-Hant.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/git-checkout-safety-and-run-lifecycle

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