Release Plan Dashboard: Add filter by Release Plan Type (Private Preview / Public Preview / GA)

## Problem

The Release Plan Dashboard (https://aka.ms/azsdk/releaseplan-dashboard) currently supports filtering by Plane (data-plane/management), Month, and Sort order. However, there is no way to filter by **release plan type** (Private Preview, Public Preview, GA).

This makes it difficult for PMs and service teams to quickly find release plans at a specific lifecycle stage, especially when the total count is high (e.g., 186 plans).

## Proposed Solution

Add a new dropdown filter alongside the existing Plane/Month filters with the following options:
- **All Types** (default)
- **Private Preview**
- **Public Preview**
- **GA**

## Acceptance Criteria

- [ ] New dropdown filter appears in the filter bar (next to "All Planes" / "All Months")
- [ ] Selecting a type filters the visible release plan cards to only show plans of that type
- [ ] Summary counts at the top update to reflect the filtered results
- [ ] Filter state is compatible with existing Plane and Month filters (they compose together)

## Context

The dashboard already shows the release type on each card (e.g., "Private Preview release plan for X"), so the data is available — it just needs an aggregated filter control.

Open the Release Plan Dashboard and locate the existing Plane and Month filter controls and the summary-count logic. Trace how release type is represented on each plan card, then add the filter alongside the existing controls and verify that type, Plane, and Month selections compose while the visible cards and summary counts update.

Written by the indexing model from the issue text.