Implement Wails desktop wrapper (per ADR 0001)

[ojongerius]

Implementation tracker for ADR 0001 — adopting Wails v2 as a desktop wrapper alongside the existing CLI mode.

Goal

Ship a native desktop build (Dashboard.app / .exe / .deb) that wraps the existing internal/server HTTP server and adds a native file-picker for .db files, while preserving the current ./dashboard -db <path> CLI flow unchanged.

Scope

Must have

• Add Wails v2 project scaffolding under a new entry point (e.g. cmd/dashboard-desktop/), reusing internal/server and internal/store
• On launch, Wails starts the HTTP server on a loopback port and loads http://127.0.0.1:<port>/ in the webview
• Native file-open dialog (Wails binding) for selecting a .db file — passes the path to the same read-only store.Open() flow
• Application menu (File → Open…, Quit) on macOS; equivalents on Windows/Linux
• Vendor Tailwind: replace the CDN <script> with a built CSS file (Tailwind CLI) embedded via //go:embed so the desktop build works offline
• Confirm internal/store remains read-only — no new write paths introduced by the Wails layer
• Existing CLI mode (./dashboard -db ... -port ...) continues to work unchanged
• Smoke-test the htmx UI on all three webview engines: WebKit (macOS), WebView2 (Windows), WebKitGTK (Linux)

Build & CI

• make desktop (or equivalent) target that invokes wails build
• GitHub Actions matrix job building Wails targets for macOS / Windows / Linux
• Document signing/notarization requirements in CONTRIBUTING.md (even if not automated yet)

Nice to have (follow-up issues OK)

• Double-click a .db file to open in Dashboard (OS file association)
• Auto-update mechanism (no first-party Wails support — evaluate community tooling)
• Recent-files menu

Out of scope

• Auto-update integration (tracked separately if pursued)
• Rewriting htmx endpoints as Wails bindings — the ADR commits to keeping HTTP
• Any change to receipt database schema or write behavior

Acceptance criteria

1. wails dev launches a window that renders the current htmx UI identically to the browser version
2. Opening a .db via the native dialog shows the same receipt list/detail views as ./dashboard -db <path>
3. go test ./... still passes
4. CLI binary from go build ./cmd/dashboard is unchanged in behavior
5. No network requests leave the machine when the desktop app is running offline (validates Tailwind vendoring)

References

• ADR 0001: docs/adr/0001-wails-desktop-wrapper.md
• Wails v2 docs: https://wails.io/docs/introduction