style(breakpoints): consolidate 9 ad-hoc values into 5 SCSS tokens

Adds src/styles/_breakpoints.scss with five canonical tokens
(\$bp-sm 600, \$bp-md 800, \$bp-lg 1024, \$bp-xl 1280, \$bp-2xl 1600)
and migrates all 15 @media (max-width: ...) occurrences across 9 files.
Configures vite.config.ts with scss loadPaths so any file can
@use "breakpoints" without relative paths.

Conservative rounding-up where existing values were near-duplicates:
500 and 512 collapse to 600, 768 collapses to 800, 1000 collapses to
1024. Slightly more viewports get the smaller-layout treatment in
those bands, which is usually beneficial on cramped widths.

Converts 7 Svelte files from <style> to <style lang="scss"> to access
the SCSS tokens. SCSS is a CSS superset, so existing rules stay valid.

1 files changed, 4 insertions, 2 deletions

diff --git a/src/routes/+page.svelte b/src/routes/+page.svelte
index ad3971d8..218bc8d5 100644
--- a/src/routes/+page.svelte
+++ b/src/routes/+page.svelte
@@ -331,7 +331,9 @@ onDestroy(() => {
   </div>
 {/if}
 
-<style>
+<style lang="scss">
+  @use "breakpoints" as *;
+
   .balanced-list-flow {
     display: grid;
     grid-template-columns: repeat(var(--balanced-list-columns), minmax(0, 1fr));
@@ -355,7 +357,7 @@ onDestroy(() => {
     margin: 0;
   }
 
-  @media (max-width: 800px) {
+  @media (max-width: $bp-md) {
     .balanced-list-flow {
       grid-template-columns: 1fr;
     }