Exportable diagnostic report file for user-shareable debugging

Follow-up to #364 / PR #372.

### Problem

When users hit issues, we often need to follow up with them to get environment details. Datadog telemetry helps for opted-in users, but we also need a way for users to manually generate and share a diagnostic report — especially for users who have telemetry disabled or for issues that need deeper context than what fits in RUM events.

### Proposal

Add a "Generate Diagnostic Report" action (in Settings or via a menu) that creates a single JSON or text file the user can share with us. This piggybacks directly off the infrastructure built in PR #372:

**Already available (reuse from existing code):**
- get-system-info: GPU vendor/label, driver version, platform, arch, OS version, RAM, CPU
- get-installation-dd-context: Installation metadata, full snapshot history with diffs
- listSnapshots + diffSnapshots: Complete environment change history

**Additional data to include:**
- App version and Electron/Chrome versions
- All installation records (names, variants, update channels, source IDs — no paths)
- Current running sessions (which installations are active)
- Recent app logs (last N lines from the Electron main process log, scrubbed of PII)
- Disk space for each installation
- ComfyUI args configuration per installation

**PII scrubbing:**
- Apply the existing scrubPII() to all string fields before writing
- Strip absolute paths entirely or redact usernames
- No environment variable values
- No model names or workflow content

**UX:**
- Button in Settings: "Export Diagnostic Report"
- Opens a save dialog, writes a .json file
- Could also be triggered via a CLI flag or keyboard shortcut for cases where the UI is broken

### Why not just use Datadog?

- Not all users opt in to telemetry
- The 256 KB RUM event limit constrains what we can attach
- A diagnostic file can include much more detail (full logs, all installations, disk info)
- Users can review exactly what they are sharing before sending it

Review PR #372 and the existing get-system-info, get-installation-dd-context, listSnapshots, diffSnapshots, and scrubPII infrastructure first. Trace how Settings actions open save dialogs and how Electron main-process logs and installation data are accessed. Done means a Settings action writes a reviewable .json report containing the requested details while applying the stated PII exclusions.

Written by the indexing model from the issue text.