feat(button): add link variant and polymorphic <a> rendering

When :href is provided, the button renders as <a> instead of <button>,
enabling navigation links with full button styling.

New :link variant renders as a minimal text link (underlined, accent
color, no background/padding) — works as both <button> and <a>.

CSS includes reset styles for a.btn (removes default link decoration)
and preserves underline for a.btn-link.

dev/squint/src/dev/squint.cljs

@@ -13,7 +13,8 @@
             [ui.switch :as switch]
             [ui.tooltip :as tooltip]
             [ui.breadcrumb :as breadcrumb]
-            [ui.pagination :as pagination]))
+            [ui.pagination :as pagination]
+            [ui.form :as form]))
 
 (defn toggle-theme! [_e]
   (let [el (.-documentElement js/document)
@@ -44,7 +45,12 @@
     (into [:div {:style {"display" "flex" "gap" "0.75rem" "flex-wrap" "wrap" "align-items" "center"}}]
           (map (fn [v]
                  (button/button {:variant v :disabled true} (str v " disabled")))
-               button-variants))))
+               button-variants))
+    (into [:div {:style {"display" "flex" "gap" "0.75rem" "flex-wrap" "wrap" "align-items" "center"}}]
+          [(button/button {:variant "primary" :href "#"} "Link primary")
+           (button/button {:variant "secondary" :href "#"} "Link secondary")
+           (button/button {:variant "link"} "Link button")
+           (button/button {:variant "link" :href "https://example.com"} "Link with href")])))
 
 ;; ── Alert ───────────────────────────────────────────────────────────
 (defn alert-demo []
@@ -175,6 +181,53 @@
     (pagination/pagination {:current 3 :total 5
                             :on-click (fn [p] (js/console.log (str "Page: " p)))})))
 
+;; ── Form ────────────────────────────────────────────────────────
+(defn form-demo []
+  (section "Form"
+    [:form {:style {"max-width" "480px"}}
+     (form/form-field {:label "Name"}
+       (form/form-input {:type "text" :placeholder "Enter your name"}))
+     (form/form-field {:label "Email"}
+       (form/form-input {:type "email" :placeholder "you@example.com"}))
+     (form/form-field {:label "Password" :hint "At least 8 characters"}
+       (form/form-input {:type "password" :placeholder "Password"}))
+     (form/form-field {:label "Select"}
+       (form/form-select {:placeholder "Select an option"
+                          :options [{:value "a" :label "Option A"}
+                                    {:value "b" :label "Option B"}
+                                    {:value "c" :label "Option C"}]}))
+     (form/form-field {:label "Message"}
+       (form/form-textarea {:placeholder "Your message..."}))
+     (form/form-field {:label "Disabled"}
+       (form/form-input {:type "text" :placeholder "Disabled" :disabled true}))
+     (form/form-field {:label "File"}
+       (form/form-file {}))
+     (form/form-field {:label "Date and time"}
+       (form/form-input {:type "datetime-local"}))
+     (form/form-field {:label "Date"}
+       (form/form-input {:type "date"}))
+     (form/form-checkbox {:label "I agree to the terms"})
+     (form/form-radio-group {:label "Preference"
+                              :radio-name "pref"
+                              :options [{:value "a" :label "Option A"}
+                                        {:value "b" :label "Option B"}
+                                        {:value "c" :label "Option C"}]})
+     (form/form-field {:label "Volume"}
+       (form/form-range {:min 0 :max 100 :value 50}))
+     (button/button {:variant "primary" :attrs {:type "submit"}} "Submit")]
+    ;; Input group
+    [:div {:style {"max-width" "480px" "margin-top" "1.5rem"}}
+     [:h4 {:style {"margin-bottom" "0.75rem"}} "Input group"]
+     (form/form-group {}
+       (form/form-group-addon {} "https://")
+       (form/form-input {:placeholder "subdomain"})
+       (button/button {:variant "primary" :size "sm"} "Go"))]
+    ;; Validation error
+    [:div {:style {"max-width" "480px" "margin-top" "1.5rem"}}
+     [:h4 {:style {"margin-bottom" "0.75rem"}} "Validation error"]
+     (form/form-field {:label "Email" :error "Please enter a valid email address."}
+       (form/form-input {:type "email" :error true :value "invalid-email"}))]))
+
 ;; ── App ─────────────────────────────────────────────────────────────
 (defn app []
   [:div {:style {"max-width" "800px" "margin" "0 auto"}}
@@ -197,7 +250,8 @@
    (switch-demo)
    (tooltip-demo)
    (breadcrumb-demo)
-   (pagination-demo)])
+   (pagination-demo)
+   (form-demo)])
 
 (defn init! []
   (eu/render (app) (js/document.getElementById "app")))