feat(frontend): Trends view consuming GET /api/analytics/trends #30

Problem

The savings-snapshot historical time-series is now collected and exposed via
GET /api/analytics/trends (PR for LeanerCloud/cloud-commitments-cli#1023/#1033), but there is no frontend view
consuming it. Users cannot yet see coverage %, utilization, committed spend, or
realized savings over time by account/provider/service.

Proposed work

Add a basic "Trends" view in frontend/src that consumes
/api/analytics/trends:

• Render the monthly series (coverage %, committed spend, usage, realized
  savings) and the by-provider / by-service breakdowns.
• Account scoping: pass the top-bar chip account_id through to the endpoint
  (the backend already enforces allowed_accounts scope).
• Escape every API field rendered into any innerHTML template via
  escapeHtml() (feedback_innerhtml_xss).
• Install event listeners once per element (stored ref / AbortController), never
  re-add on a repeatedly-called render (feedback_event_listener_dedup).
• Do NOT touch frontend/src/dashboard.ts ServiceSavings or
  handler_dashboard.go / types.go ServiceSavings (owned elsewhere).

References

• Endpoint + response shape: internal/api/handler_analytics.go
  (getAnalyticsTrends, TrendsResponse).
• Backend feature: PR closing fix(analytics): guard div-by-Term, zero coverage/usage, cloud_account_id scoping & VARCHAR(20) before wiring collector cloud-commitments-cli#1023 / chore(analytics): finish-or-remove decision for dead analytics collector/Query* surface (no-delete: needs human call) cloud-commitments-cli#1033.

Filed as the deferred frontend follow-up noted in the analytics historical-savings PR.