fix(errors): emit JSON envelope from WriteHTTPError + harden fetch callers

[sarg3nt]

Summary

• errors.WriteHTTPError historically wrote text/plain via http.Error. Any API handler routing errors through it (Logs, Services, Certificates, …) sent bodies like Failed to fetch logs. Please try again later.. Frontend JS that unconditionally response.json()s the body threw Unexpected token 'F', "Failed to "... is not valid JSON, which the Logs/Services pages rendered as the hostile error toast users were seeing.
• Change WriteHTTPError to emit the same {"success": false, "message": "..."} envelope Handler.jsonError produces, with Content-Type: application/json. Both helpers now share a wire format, and JS callers can parse error responses the same way they parse successes.
• Harden the two visibly-affected pages (Logs, Services) so they check response.ok, prefer the JSON envelope's .message for the toast, and fall back to response.statusText for non-JSON bodies. (The catch handler in logs.templ now uses textContent / replaceChildren instead of building HTML, sidesteps the existing innerHTML pattern lint.)
• Add a regression test in internal/framework/errors that asserts the new wire format.

Phase 1 slice of #112.

Test plan

• go test -count=1 ./internal/framework/errors/... ./internal/framework/handler/... clean (new errors_writehttp_test.go + existing os_updates_error_test.go)
• go vet ./... clean
• go build ./... clean
• On a live deployment, navigate to /logs and /services for a box where the agent can't serve those gears; confirm the error toast surfaces the agent's actual message (e.g. "Failed to fetch logs. Please try again later.") instead of Unexpected token 'F'.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR standardizes WriteHTTPError responses as JSON envelopes and updates Logs/Services frontend fetch handling to display backend error messages instead of JSON parse failures.

Changes:

• WriteHTTPError now emits {"success": false, "message": "..."} with application/json.
• Logs and Services page fetch paths now check response.ok and extract JSON error messages.
• Added regression tests for the new WriteHTTPError JSON envelope.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
gearbox/internal/framework/errors/errors.go	Changes shared HTTP error serialization to JSON.
gearbox/internal/framework/errors/errors_writehttp_test.go	Adds tests for JSON error response shape.
gearbox/internal/framework/templates/pages/logs.templ	Hardens log fetch error handling and safer DOM update on failure.
gearbox/internal/framework/templates/pages/services.templ	Hardens initial services fetch error handling.