Show the build in a footer, linked to its release or commit

The version moves from the header to a "Powered by Jarl" footer. A build
of a release tag (HEAD tagged v<VERSION>, no tracked changes; the release
workflow sets RELEASE_TAG) shows and links the release; any other build
shows and links its commit, marked -modified with uncommitted changes.
jarld version, jarl --version, /status and /auth/whoami report the same.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/types.go

@@ -8,6 +8,7 @@ import (
 
 	"jarl/internal/model"
 	"jarl/internal/nscheck"
+	"jarl/internal/version"
 )
 
 // Version is the API path prefix.
@@ -92,6 +93,8 @@ type Whoami struct {
 	Permissions []string `json:"permissions"`
 	Admin       bool     `json:"admin"`
 	Version     string   `json:"version"`
+	// Build is what the server was built from (release or commit).
+	Build version.Build `json:"build"`
 }
 
 // LoginRequest logs in.
@@ -156,6 +159,7 @@ type Changeset struct {
 // StatusResponse is GET /status.
 type StatusResponse struct {
 	Version string          `json:"version"`
+	Build   version.Build   `json:"build"`
 	Engine  json.RawMessage `json:"engine"`
 	Time    time.Time       `json:"time"`
 	// ExpiringTokens are tokens that expire within TokenExpiryWarning or