Enhance map rendering with WebP support and loading feedback

- Update tile rendering to support both PNG and WebP formats.
- Introduce a loading spinner and progress bar for build feedback.
- Remove the manual build button; builds now trigger automatically on map selection.
- Add a phase plan document outlining future improvements and goals.

map_renderer/phase-plan.md

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+# Map Renderer Phase Plan
+
+## Phase 1
+
+Goal: tighten the current viewer flow without changing the map semantics.
+
+- remove the explicit manual build button because map selection and filter changes already trigger builds automatically
+- hide the viewport's "choose a detected map" message as soon as a map is selected and a build starts
+- add build feedback in the side panel with a spinner and a simple phase-based loading bar
+- remove visible tile seam lines and the synthetic background grid so the map reads as one surface
+- use a more efficient streamed visualization format for interactive tiles while keeping PNG as the final export format
+
+Phase 1 implementation choice:
+
+- interactive tiles switch from PNG to lossless WebP because it is broadly supported in current browsers and is more bandwidth-efficient for repeated server-rendered tile delivery
+- full-map download remains PNG so export quality and compatibility stay unchanged
+
+## Phase 2
+
+Goal: promote editor-only content from "baked into the raster" to interactive overlay objects.
+
+- keep the base map rendered as a flat server-generated tile surface
+- extract editor-only objects into a standalone overlay data stream
+- render those overlay items in the client as positioned interactive markers or sprites above the base map
+- on hover, slightly scale the hovered item and show a tooltip with its decoded metadata payload
+- improve roof detection before or during the overlay split because the current roof filtering still lets some roofs render when they should not
+- identify occluding helper geometry such as invisible walls and render those semitransparently so they remain legible without hiding too much of the map beneath them
+- fix pipe rendering because pipes currently are not showing up correctly
+- investigate force-field rendering because they appear yellow instead of the expected blue semitransparent look; this may be a debug-shape choice issue or a palette/color-rendering issue
+- likely revisit ScummVM Crusader handling in `D:\source\scummvm` to confirm what editor/debug records carry and how best to decode them for display
+
+Open questions for phase 2:
+
+- which editor-only families should become interactive overlays versus remain baked into the base render
+- what exact metadata fields are reliable enough to expose in the tooltip
+- whether some editor-only entries should be clustered, filtered, or toggled by family to keep dense maps usable