feat: Saved Responses sidebar panel with search, filter, and CRUD

[markkr125]

Summary

Adds a fully self-contained Saved Responses panel to the right sidebar, allowing users to browse, search, filter, rename, duplicate, and delete saved response examples without leaving the main editor view.

Changes

New: saved_responses sub-package (saved_responses)

• panel.py — SavedResponsesPanel with list/detail splitter layout. Selecting a saved response displays its body (with Pretty/Raw toggle), headers, and request snapshot inline.
• delegate.py — Custom painted list delegate with coloured status-code badge, response name, and metadata line (date, language, size).
• search_filter.py — _PanelSearchFilterMixin adding body search (Ctrl+F, match highlighting, prev/next navigation) and structured filter (JSONPath for JSON, XPath for XML).
• helpers.py — Extracted formatting utilities: body size formatting, language detection, JSON/XML pretty-printing, header rendering.

Service & repository layer

• CollectionService: new static methods — get_saved_responses(), get_saved_response(), save_response(), rename_saved_response(), delete_saved_response(), duplicate_saved_response().
• SavedResponseDict TypedDict for normalized saved response payloads.
• Repository: new functions for rename, delete, duplicate, and query (ordered by created_at DESC).

Sidebar integration

• RightSidebar gains a floppy-disk icon rail button that toggles the Saved Responses flyout panel.
• MainWindow wires panel signals (save_current_requested, rename_requested, duplicate_requested, delete_requested, refresh_requested) and refreshes the panel on tab switch and after HTTP send.
• ResponseViewerWidget emits save_availability_changed(bool) so the sidebar can enable/disable "Save Current".

UI polish

• Selected list items now use palette.highlightedText() for both name and metadata text (fixes invisible text on selection).
• status_color() added to theme for status-code badge colouring.
• Global QSS rules added for new objectName selectors (savedResponseStatusBadge).

Tests

• 455 new lines in test_saved_responses_panel.py covering panel rendering, selection, context menus, search, filter, empty states, and delegate painting.
• 77 new lines in test_service.py for saved response CRUD service methods.
• Updated test_response_viewer.py and test_sidebar.py for new panel integration.