feat(handler): unify ?server= / ?box_id= via resolveActiveBox helper

[sarg3nt]

Summary

Three resolution conventions co-existed pre-#112 Phase 4:

1. ?server=<id> — gear-settings deep links and the OS-Updates page.
2. ?box_id=<id> — InjectIntegrationStatus middleware (header-pill URL persistence).
3. gearbox_active_box cookie — sticky pill selection.

resolveBoxIDFromRequest only knew ?server=, so handlers that followed the middleware convention lost pill-aware routing.

• resolveBoxIDFromRequest now accepts both ?server= and ?box_id= as synonyms (precedence: server > box_id > cookie > default).
• New resolveActiveBox(r) returns the full *models.BoxConfig + presence bool so handlers that need an agent URL or API key don't chase a separate getServerConfig call. Folds the "single helper for active box" invariant from the issue proposal into one call site.

Phase 4 of #112. Independent of the other open PRs — branches off main.

Test plan

• go build ./... and go vet ./... clean
• go test -count=1 ./internal/framework/handler/... clean — four new tests (TestResolveBoxIDFromRequest_*, TestResolveActiveBox_ReturnsServerConfig)
• On a deployment, switch the active box via the header pill (writes the cookie), then navigate to a deep-link with neither ?server= nor ?box_id= (e.g. /gears) — confirm the cookie's box is honored.
• Hit a deep-link with ?box_id=other-box — confirm it overrides the cookie just like ?server= does.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR standardizes “active box” resolution across dashboard handlers by treating ?server= and ?box_id= as synonyms (with defined precedence) and introduces a resolveActiveBox(*http.Request) helper to return the resolved *models.BoxConfig directly. This supports the broader capability-driven rendering work by making box selection consistent between deep links, middleware-persisted selection, and the active-box cookie.

Changes:

• Extend resolveBoxIDFromRequest to accept both ?server= and ?box_id= with precedence server > box_id > cookie > default.
• Add resolveActiveBox(r) helper to return the resolved *models.BoxConfig plus a presence boolean.
• Add unit tests covering the new query-param alias behavior and the new helper’s happy path.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
gearbox/internal/framework/handler/handler.go	Accepts box_id as an alias for server in request resolution and adds the resolveActiveBox helper.
gearbox/internal/framework/handler/resolve_box_test.go	Adds unit tests for resolver precedence and resolveActiveBox returning the expected BoxConfig.