fix: scale-aware viewport units and restore/extend Open editor

[b404dev]

Summary

Two fixes bundled together, both regressions/gaps in the last release:

1. Text & UI scale looked "horrible" / didn't shift properly
The scale control transforms .shell, but the stylesheets use vh/vw in ~60 places (modals, the code pane, splash screen, responsive clamps). Those units always measure against the real, unscaled window regardless of an ancestor's transform, so at any scale other than 100% those elements stopped lining up with everything else that scales via the transform. Fixed by introducing --vh/--vw custom properties (1% of the true viewport, divided by the active scale) and rewriting every vh/vw literal to use them via calc(), so viewport units now scale in lockstep with the rest of the UI.

2. Open editor still doesn't work on macOS
Root cause turned out to be two separate things:

• PR fix: resolve git and gh from GUI-launch locations #14 merged an outdated commit of its own branch — gh pr view 14 --json commits shows only 2 commits, not the 3rd one (a56d3d8) that actually contained the ExecutablePath fix for OpenInEditor. It shipped in the v0.1.15 changelog and its own now-orphaned branch, but never actually reached main. Restored here.
• Even with that fix, an editor configured as code with no CLI shim installed (VS Code's "Shell Command: Install 'code' command in PATH" never run) has no binary to find anywhere — ExecutablePath can't invent one. Added a macOS-only fallback: when the binary truly can't be found, launch the application directly via open -a "<App Name>" for a short list of common editors (VS Code, VS Code Insiders, Cursor, Sublime Text, Zed, WebStorm, IntelliJ IDEA, Atom), forwarding any extra configured flags via --args.

Test plan

• go build ./backend/..., go vet ./backend/...
• go test ./backend/... — restored TestOpenInEditorUsesExecutablePathFallbacks (lost the same way as the fix itself) and added TestMacEditorLaunchArgumentsResolvesKnownApplications / TestMacEditorLaunchArgumentsForwardsExtraFlags; the 4 other failing tests are pre-existing/environment-dependent (present on main too)
• npx tsc -b, npm run build, npm test all clean
• make build (real wails build) succeeds end-to-end
• Not visually verified in a running desktop session (no display available in this environment) — please confirm the scale slider now feels consistent across its range, and that Open editor launches VS Code