fix(app): fix trace copy to include user input + add session-level Export trace

[jeonghun-jj-lee]

Important

Problem — The per-turn "Copy trace" button at the bottom of each message only copies the assistant's output, omitting the user's input that triggered it. There is also no way to export a full session trace — users must copy turn by turn. Both gaps make it harder to share, review, or archive session transcripts.

Approach — (1) Extend buildTrace to accept an optional userText parameter, prepended with a You: prefix when present. (2) Add a buildSessionTrace function that walks all turns in order. (3) Fix both per-turn copy call sites to include the user input. (4) Add an "Export trace" menu item to the session three-dot menu that saves the full session trace to a file.

Scope — in: per-turn copy trace fix (both V2 timeline and session-ui TurnFooter paths), session-level export trace menu item (both old DropdownMenu and V2 MenuV2 layouts), buildTrace + buildSessionTrace functions and tests · out: share feature, cloud export, format options (markdown/JSON), reasoning-part inclusion

Acceptance Criteria

• Clicking the per-turn copy trace button copies the user's input (prefixed with You: ) followed by the assistant trace, separated by a blank line
• User messages with empty or whitespace-only text produce no You: prefix line
• The session three-dot menu contains an "Export trace" item positioned after Share and before Archive
• Clicking "Export trace" saves the full session trace to a .md file via the platform save-file dialog (desktop) or <a download> fallback (web/webview)
• The session trace concatenates all turns in order: You: <input>\n\n<assistant trace> per turn, separated by blank lines
• buildTrace existing callers that pass no userText produce identical output to before (backward compatible)
• buildSessionTrace handles edge cases: sessions with no messages, turns with no assistant response, user messages with no text part
• Existing build-trace.test.ts tests continue to pass; new tests cover userText prepend, session-level trace, and edge cases

Testing Decisions

Extend the existing build-trace.test.ts suite in packages/session-ui — it already covers the segment-joining logic and skip-tool filtering. Add cases for the userText parameter and a new describe block for buildSessionTrace.

Key Decisions

• User prefix is You: , no assistant prefix — keeps the trace clean and readable; the assistant's output is obvious from context.
• Per-turn = clipboard, session-level = file save — a single turn is small enough for the clipboard; a full session can be large and is better saved to disk.
• File save uses existing platform bridge — saveFilePickerDialog is already wired in the desktop Electron app (IPC through save-file-picker); the web/webview fallback uses <a download> with a Blob URL. No new bridge messages needed.
• Menu item position: after Share, before Archive — groups content actions (Rename, Share, Export) together, with Archive as transition to the destructive zone (Delete).
• Reuse ui.message.copyTrace i18n key for per-turn; add a new key for the session-level "Export trace" — the actions are different enough to warrant distinct labels.

Constraints & Invariants

• buildTrace signature change must be backward-compatible (new parameter is optional, trailing)
• TRACE_SKIP_TOOLS set unchanged — read/glob/grep/list remain excluded
• The trace is plain text, not markdown — no formatting beyond the You: prefix and $ command convention for bash
• The menu item must appear in both old DropdownMenu and V2 MenuV2 layouts (gated by settings.general.newLayoutDesigns())

Prior Art

• build-trace.ts — the existing trace builder
• message-timeline.tsx lines 481-489 — copyTraceForTurn (V2 timeline path)
• message-part.tsx lines 976-1064 — TurnFooter (session-ui path)
• message-timeline.tsx lines 2110-2269 — the three-dot menu (both layouts)
• platform.tsx line 63 — saveFilePickerDialog abstraction
• desktop/src/main/ipc.ts lines 188-198 — Electron showSaveDialog handler
• session.tsx lines 1886-1905 — existing <a download> pattern for attachments

Notes

• TurnFooter in message-part.tsx currently receives only messages: AssistantMessage[] — it needs a new userMessageID (or userText) prop threaded from AssistantParts, which already has access to the user message context.
• The saveFilePickerDialog platform method is defined and wired end-to-end in desktop but has zero callers today — this will be its first consumer.
• Default filename for session export: trace-<session-title-slug>-<date>.md (or .txt).