feat: use custom Nix highlight queries for richer syntax colors

- queries/nix-highlights.scm: Adapted from Helix editor queries.
  Properly identifies functions, properties, variable parameters,
  escape sequences, and string interpolation. Much richer than
  the bundled tree-sitter-nix query (99 lines vs 113 lines).

- src/highlight.rs: Load custom Nix query via include_str!
  instead of using bundled HIGHLIGHTS_QUERY.

- static/style.css: Fix variable (#e36209) and punctuation (#586069)
  colors to be distinct from text color.

Results: hl-function now appears (6 occurrences), hl-property
visible (17), hl-variable reduced from 43 to 17. Functions like
mkShell now render in purple/blue instead of orange.

queries/nix-highlights.scm

@@ -0,0 +1,133 @@
+; Nix syntax highlighting queries (adapted from Helix editor)
+; Maps to standard tree-sitter capture names for compatibility with tree-sitter-highlight
+
+(comment) @comment
+
+; Punctuation
+[
+  ";"
+  "."
+  ","
+  "="
+  ":"
+  (ellipses)
+] @punctuation.delimiter
+
+[
+  "("
+  ")"
+  "["
+  "]"
+  "{"
+  "}"
+] @punctuation.bracket
+
+; Keywords
+"assert" @keyword
+"or" @keyword
+"rec" @keyword
+
+[
+  "if" 
+  "then"
+  "else"
+] @keyword
+
+[
+  "let"
+  "inherit"
+  "in"
+  "with" 
+] @keyword
+
+; Variables and identifiers
+(variable_expression name: (identifier) @variable)
+
+; Attribute access (properties)
+(select_expression
+  attrpath: (attrpath attr: (identifier)) @property)
+
+; Function calls
+(apply_expression
+  function: [
+    (variable_expression name: (identifier) @function)
+    (select_expression
+      attrpath: (attrpath
+        attr: (identifier) @function .))])
+
+; Builtin variables
+((identifier) @variable.builtin
+ (#match? @variable.builtin "^(__currentSystem|__currentTime|__nixPath|__nixVersion|__storeDir|builtins)$")
+ (#is-not? local))
+
+; Builtin functions
+((identifier) @function.builtin
+ (#match? @function.builtin "^(__add|__addErrorContext|__all|__any|__appendContext|__attrNames|__attrValues|__bitAnd|__bitOr|__bitXor|__catAttrs|__compareVersions|__concatLists|__concatMap|__concatStringsSep|__deepSeq|__div|__elem|__elemAt|__fetchurl|__filter|__filterSource|__findFile|__foldl'|__fromJSON|__functionArgs|__genList|__genericClosure|__getAttr|__getContext|__getEnv|__hasAttr|__hasContext|__hashFile|__hashString|__head|__intersectAttrs|__isAttrs|__isBool|__isFloat|__isFunction|__isInt|__isList|__isPath|__isString|__langVersion|__length|__lessThan|__listToAttrs|__mapAttrs|__match|__mul|__parseDrvName|__partition|__path|__pathExists|__readDir|__readFile|__replaceStrings|__seq|__sort|__split|__splitVersion|__storePath|__stringLength|__sub|__substring|__tail|__toFile|__toJSON|__toPath|__toXML|__trace|__tryEval|__typeOf|__unsafeDiscardOutputDependency|__unsafeDiscardStringContext|__unsafeGetAttrPos|__valueSize|abort|baseNameOf|derivation|derivationStrict|dirOf|fetchGit|fetchMercurial|fetchTarball|fromTOML|import|isNull|map|placeholder|removeAttrs|scopedImport|throw|toString)$")
+ (#is-not? local))
+
+; Strings
+[
+  (string_expression)
+  (indented_string_expression)
+] @string
+
+; Special string types (paths, URIs)
+[
+  (path_expression)
+  (hpath_expression)
+  (spath_expression)
+] @string.special.path
+
+(uri_expression) @string.special.uri
+
+; Boolean constants
+((identifier) @constant.builtin (#match? @constant.builtin "^(true|false)$"))
+
+; Null constant
+((identifier) @constant.builtin (#eq? @constant.builtin "null"))
+
+; Numbers
+(integer_expression) @number
+(float_expression) @number
+
+; Escape sequences
+(escape_sequence) @escape
+(dollar_escape) @escape
+
+; Function parameters
+(function_expression
+  "@"? @punctuation.delimiter
+  universal: (identifier) @variable.parameter
+  "@"? @punctuation.delimiter
+)
+
+(formal
+  name: (identifier) @variable.parameter
+  "?"? @punctuation.delimiter)
+
+; String interpolation
+(interpolation
+  "${" @punctuation.special
+  "}" @punctuation.special) @embedded
+
+; Operators
+(unary_expression
+  operator: _ @operator)
+
+(binary_expression
+  operator: _ @operator)
+
+; Attribute bindings
+(binding
+  attrpath: (attrpath attr: (identifier)) @property)
+
+; Inherit expressions
+(inherit_from attrs: (inherited_attrs attr: (identifier) @property))
+(inherited_attrs attr: (identifier) @variable)
+
+; Has attribute expression
+(has_attr_expression
+  expression: (_)
+  "?" @operator
+  attrpath: (attrpath
+    attr: (identifier) @property))