Add colorless filters, and make set listings filterable

Colorless

There was no way to ask for colorless cards — the colour toggles are a 5-bit
WUBRG mask where 0 already means "don't filter by colour", leaving nothing to
express "no colours at all". That's exactly what you need to find a colorless
commander's card pool.

Adds a "C" pip to both colour rows, matching how ColorPips already renders
colorless cards, backed by two explicit booleans rather than a sixth bit — a
filter mask and a card's real colour mask staying the same shape is worth
keeping. The two rows mean different things and both are useful: on the
commander row C filters colour *identity*, which is the Commander rule; on the
includes row it filters the card's own colours. A card printed colorless can
still carry a coloured identity through mana symbols in its text, and it's the
identity that bars it from a colorless commander's deck.

Set listings

News > Releases had its own card path and so couldn't be filtered. A new
set_code filter (matched against printings.set_code, already ingested) lets the
set browser run through search_reference instead, so every criterion works
there identically rather than being reimplemented over a live Scryfall
response.

That alone would have cost the ability to browse an upcoming set, since
search_reference only knows downloaded card data — and upcoming sets are half
the point of the tab. So the browser picks a source per set: downloaded data
where it exists, with the full filter panel; otherwise Scryfall directly, with
the panel hidden and a line explaining why. A panel that silently does nothing
would be worse than no panel.

The source is decided by an *unfiltered* probe. Keying off whether the current
criteria match anything would flip a properly indexed set to "live" the moment
a filter excluded everything, dropping the panel exactly when it was in use.

crates/mtgc-core/src/model.rs

@@ -314,6 +314,20 @@ pub struct LibraryFilter {
     /// Only cards that can head a Commander deck.
     #[serde(default)]
     pub can_be_commander: bool,
+    /// Only cards with an empty colour identity — what a colourless commander
+    /// can actually play. The colour bitmasks can't express this: 0 there means
+    /// "no filter", not "no colours".
+    #[serde(default)]
+    pub colorless_identity: bool,
+    /// Only cards printed in this set (matched against `printings.set_code`).
+    /// Lets the release browser reuse the whole criteria pipeline instead of
+    /// reimplementing it over a live Scryfall response.
+    pub set_code: Option<String>,
+    /// Only cards that are themselves colourless (`colors` empty). Distinct
+    /// from `colorless_identity`: a card can be colourless yet carry a coloured
+    /// identity through mana symbols in its rules text.
+    #[serde(default)]
+    pub colorless_colors: bool,
     /// Printed power/toughness range. Cards whose printed value isn't a plain
     /// number ("*", "1+*") are excluded from these, since they can't be
     /// compared — see `oracle_cards.power`.