web-player: buttons (Button, IconButton, PlayButton)

Button: subtle variants (primary/secondary, left unchanged per the
user's call), glass (formerly secondaryOnGlow, with its own
font-normal), accent, strong (cta-strong/ink, replacing the `!`
overrides on the Home hero), danger. Uses the --color-btn* tokens
instead of bg-[var(--mf-btn-...)]. Radius unified to rounded-control.

IconButton: adds shape (square/round), size (xs/sm/md/lg), tone
(muted/subtle/plain), pressed (aria-pressed + text-accent),
href/reload (links), surface (permanent background + hover to
surface-hover instead of the generic `hover` alias, which matched
surface-2 and gave no contrast).

New PlayButton: a single place for the play/pause SVGs, strong/glass
variants, as="span" for non-interactive overlays. Replaces
TransportControls' main button, the MixTile overlay, and the "Top
result" circle in Explore.

Migrated to IconButton: close queue, "Create playlist" (Sidebar),
Like (TrackInfo, with pressed), queue (PlayerExtras), back (TopBar),
login (MobileHeader), and the row actions in tables (remove/add, size
xs). BackLink uses size-icon-sm and drops a duplicate hover.

Decision (not blocking): remove TopBar's decorative "forward" button
and PlayerExtras' decorative AlignJustify icon (visually dead code).
TransportControls' buttons (shuffle/prev/next/repeat) are
deliberately left unmigrated: they currently have no background or
hit-box of their own, and IconButton always applies a fixed-size
box — migrating them would have added a visible hover pill that
couldn't be verified without Docker this session.

Verified with npm run check/build.

web-player/src/routes/(app)/albums/[id]/components/AlbumTrackTable.svelte

@@ -54,7 +54,7 @@
 							update()}
 				>
 					<input type="hidden" name="trackId" value={track.id} />
-					<IconButton type="submit" label="Quitar del álbum" size="sm" revealOnHover>
+					<IconButton type="submit" label="Quitar del álbum" size="xs" revealOnHover>
 						<X class="h-3.5 w-3.5" />
 					</IconButton>
 				</form>

web-player/src/routes/(app)/albums/[id]/components/LibraryPicker.svelte

@@ -55,7 +55,7 @@
 							update()}
 				>
 					<input type="hidden" name="trackId" value={track.id} />
-					<IconButton type="submit" label="Añadir al álbum" size="sm" revealOnHover>
+					<IconButton type="submit" label="Añadir al álbum" size="xs" revealOnHover>
 						<Plus class="h-3.5 w-3.5" />
 					</IconButton>
 				</form>