docs(spec): add Electron desktop pivot design (#134)

## Summary

Adds the agreed design spec for converting game-server-deploy from a
Nest+React web app into a single-binary Electron desktop application.
Captures locked decisions, runtime architecture, cloud-provider
abstraction seams, migration plan, and the seven new epics (A–G)
decomposed across four releasable milestones.

The spec is the single source of truth that the upcoming GitHub issue
restructure will reference:
- Stale closures: #5, #6, #7, #10, #43, #103
- Repurposed epics: #80, #81, #82 (children renamed to reflect
desktop-main scope)
- Closed: #83 + child #104; viable children of #83 fold into new Epic D
- New epics: A (Electron shell), B (IPC migration), C (Cloud
abstraction), D (Local terraform orchestration), E (First-run wizard), F
(Test migration), G (Distribution)

## Test plan

- [ ] Read the spec end-to-end; check for ambiguity, contradictions, or
missing scope.
- [ ] Confirm decision table matches the conversation transcript.
- [ ] Confirm the migration plan handles every existing in-flight or
open issue.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

Author: CoderCoco