fix: wire release build info into the version the binaries report

[selimdizer]

v0.4.0 shipped reporting Commit: dev and Built: unknown. GoReleaser was injecting -X main.version/commit/date, but package main only holds func main() — the values landed nowhere. The desktop build passed no ldflags at all, and pkg/api carried its own hardcoded copy of the version, so cutting a release meant editing the number in three places.

Changes

• internal/buildinfo is now the single source of version, commit and build date. cmd/version.go and pkg/api.GetVersion both read it.
• GoReleaser and all five wails desktop builds inject into that package.
• Without ldflags, the values come from the toolchain's embedded build info (debug.ReadBuildInfo), so go install and local builds report the real revision and time instead of placeholders. A modified tree is marked -dirty.
• The update check strips build metadata before comparing, which otherwise parsed 0.4.10+dirty as 0.4.0 and could hide an available update.

Verification

$ go build -ldflags "-X .../buildinfo.version=9.9.9 -X .../buildinfo.commit=abc1234 \
    -X .../buildinfo.buildDate=2026-08-01T00:00:00Z" -o walgo .
$ ./walgo version
Walgo v9.9.9
Commit:  abc1234
Built:   2026-08-01T00:00:00Z

$ go build -o walgo .        # no ldflags — falls back to build info
$ ./walgo version
Walgo v0.4.0+dirty
Commit:  a0a860494391a84efc9e3a08a7a1fe90a3d14843-dirty
Built:   2026-07-31T22:07:32Z

go test ./... passes across all 23 packages with a cleared cache; golangci-lint v2.12.2 (the CI version) reports 0 issues. New unit tests cover both the ldflags and fallback paths.

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