fix(app): guard session-header current() against undefined when options is empty

[cyberprophet]

Issue for this PR

Closes #16459
Closes #16467

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

session-header.tsx has a current memo that can return undefined:

const current = createMemo(() => options().find((o) => o.id === prefs.app) ?? options()[0])

options() is temporarily empty during two scenarios:

1. Startup — before platform.checkAppExists resolves for each installed app
2. Project switching — race condition between cleanup and re-render

When options() is empty, options()[0] is undefined, so current() is undefined. The component then accesses current().id, current().label, and current().icon without a null check, throwing TypeError: Cannot read properties of undefined (reading 'id') and crashing the entire UI with the "Something went wrong" screen.

The fix adds a third fallback using fileManager() (which is always defined) so current() is never undefined:

const current = createMemo(
  () =>
    options().find((o) => o.id === prefs.app) ??
    options()[0] ??
    ({ id: "finder", label: fileManager().label, icon: fileManager().icon } as const),
)

This is the same pattern as PR #16475 / #16476 which fix the structurally identical crash in TextShimmer (issue #16473).

How did you verify your code works?

• Traced the crash stack trace from Desktop UI crashes on load with Cannot read properties of undefined (reading 'id') in session bundle #16459 to session-header.tsx line 306
• Confirmed options() can be empty during startup and project switching by reading the createEffect that populates exists asynchronously (lines 269–290)
• LSP diagnostics show no type errors after the change
• The fallback object matches the shape of fileManager() return value, so all downstream .id, .label, .icon accesses remain valid

Screenshots / recordings

No visual change — this only prevents a crash, the UI behavior when options() is non-empty is identical.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[cyberprophet]

E2E Linux Failure Analysis

The e2e (linux) failure is not caused by this PR's changes.

Failed test: e2e/commands/panels.spec.ts:10 › review panel can be toggled via keybind
Flaky test: e2e/session/session-undo-redo.spec.ts:131 › slash undo/redo traverses multi-step revert stack

Both failures reproduce identically on the dev branch across the last 3 consecutive commits (runs 22799518867, 22799515032, 22799345432) — none of which touch session-header.tsx.

This PR only modifies a single line in packages/app/src/components/session/session-header.tsx (the current memo fallback), which has no interaction with the review panel keybind or undo/redo logic.

Re-running CI to confirm.

[cyberprophet]

E2E Failure Confirmed Pre-existing (Not This PR)

Re-ran CI twice. Both runs fail with identical tests:

Test	Status
e2e/commands/panels.spec.ts:10 › review panel can be toggled via keybind	❌ failed (3 retries)
e2e/session/session-undo-redo.spec.ts:131 › slash undo/redo traverses multi-step revert stack	⚠️ flaky

These same tests fail on dev branch across 5 of the last 8 runs (runs 22799518867, 22799515032, 22799345432, 22798448488) — all on commits that do not touch session-header.tsx.

This PR's diff is a single-line change to the current memo in session-header.tsx. It has no code path that could affect the review panel keybind or undo/redo message visibility logic.

[jon23d]

It may be worth comparing this to jon23d#1. I compiled it and ran the binary with:

bun install
./packages/opencode/script/build.ts --single
./packages/opencode/dist/opencode-linux-x64/bin/opencode web --host 0.0.0.0

I was able to start a session successfully after that.