Add selectable site theme, custom CSS, and new font set

- Data.theme/Data.custom_css site settings with /_api/settings support;
  theme stylesheet linked per page, custom CSS inline as #pagerite-user,
  swapped during fetch-navigation
- Site editor: theme selector, custom CSS CodeMirror field; in-place
  stylesheet swap (diff-based, preserves editor stylesheets); dev swaps
  Vite-injected style tags since no <link>s exist there
- Dev theme delivery via <meta name="pagerite:theme"> + dynamic import
  in pagerite.js instead of a hardcoded purple import
- Fix dead edit pens in prod: Vite app builds strip entry exports, so
  the dynamic-imported main.js had no openEditor/closeEditor; build with
  preserveEntrySignatures 'exports-only' and log editor load failures
  instead of swallowing them
- Editor slide-in is one-shot: cleared on animationend so stylesheet
  swaps cannot restart a finished animation
- Replace Fraunces/Literata with Source Sans 3/Serif 4, Inter and
  Montserrat variable fonts; font stacks as CSS variables

frontend/src/assets/pagerite.css

@@ -12,6 +12,9 @@
   --accent: #0056b3;
   --accent2: #003d80;
   --line: #0000001a;
+  --font-body: "Source Sans 3", system-ui, sans-serif;
+  --font-heading: "Source Serif 4", Georgia, serif;
+  --font-code: "Fira Code", ui-monospace, monospace;
   /* Width of the docked editor panel (used both here for shifting the page
      and in the Vue editor's own styles). */
   --editor-w: min(46rem, 50vw);
@@ -26,9 +29,12 @@ html {
 }
 
 body {
-  font-family: "Literata", Georgia, serif;
+  font-family: var(--font-body);
   font-size: 1.05rem;
   line-height: 1.65;
+  /* Tabular numerals wherever the active font supports them; avoids
+     numbers jumping in width as counters/values change. */
+  font-variant-numeric: tabular-nums;
   margin: 0;
   background: var(--bg);
   color: var(--text);
@@ -79,7 +85,7 @@ body {
 }
 
 #brand {
-  font-family: "Fraunces", serif;
+  font-family: var(--font-heading);
   font-weight: 700;
   font-size: 2.4rem;
   text-decoration: none;
@@ -257,7 +263,7 @@ main {
 article h1,
 article h2,
 article h3 {
-  font-family: "Fraunces", serif;
+  font-family: var(--font-heading);
   font-weight: 600;
   line-height: 1.25;
 }
@@ -463,7 +469,7 @@ pre:hover .copy,
 }
 
 code {
-  font-family: "Fira Code", ui-monospace, monospace;
+  font-family: var(--font-code);
   font-size: 0.88em;
 }