Tauri GUI: functional desktop application

## Description

`gui/src-tauri/` is a scaffold: `main.rs` (13 lines) prints a demo message, `commands.rs` (62 lines) has 7 stub functions returning `{"status": "stub"}`. No Tauri window setup, no backend integration, no frontend UI.

## Requirements

Per whitepaper and spec Phase 10 (T120-T126):

- **Tauri backend bridge**: expose agent IPC commands as Tauri commands
- **Donor dashboard**: enrollment status, credit balance, trust score, caliber class, active leases, preemption history
- **Submitter dashboard**: job submission form, job list, status, result download, receipt verification
- **Governance board**: proposal list, create proposal, cast vote, view results
- **Mesh LLM status**: expert roster, router health, self-improvement task history
- **Settings page**: workload class opt-in/out, CPU cap, storage cap, OTel endpoint
- Frontend in React/TypeScript per spec

## Success Criteria

- [ ] Tauri app launches and displays functional window
- [ ] Donor dashboard shows real agent state from backend
- [ ] Job submission works end-to-end through GUI
- [ ] Governance voting works through GUI
- [ ] Settings changes persist and affect agent behavior
- [ ] WCAG 2.1 AA accessibility (keyboard navigation, screen reader, contrast)
- [ ] Internationalization framework with English + 2 additional languages
- [ ] Tested on macOS, Linux, and Windows

## Testing (Principle V)

- Launch app on each platform → verify window renders
- Submit job through GUI → verify completion
- Change settings → verify agent behavior changes
- Screen reader testing for accessibility
- Visual regression testing via screenshots