refactor(theme): remove 184 var() fallbacks — every one was unreachable

#2277 counted ~150 "raw colour literals bypassing the tokens". Measuring them
told a different story: 184 sat in `var(--token, #fallback)` position, and a
check against theme.css shows every one of those tokens IS declared. So the
fallbacks could not render. Not drift — vestigial.

They were also not this palette. The most common were Tailwind and Flat-UI
defaults — #6366f1 indigo, #22c55e green, #f59e0b amber, #3b82f6 blue,
#e74c3c and #27ae60 — a second, unsanctioned colour scheme sitting in the
codebase looking like the app's colours to anyone reading it.

Removing them is not tidying. #2319's lesson is that a fallback is WORSE than a
missing token: a missing token renders as nothing and someone eventually
notices, while a fallback renders something plausible forever. These 184 were
one token rename away from silently repainting the app in Tailwind. The design
token check would catch the rename — but the fallback is precisely the thing
that would make it invisible if the check were ever bypassed.

Literal count 152 -> 45, which matters beyond the number: a report that is
mostly unreachable noise is one people stop reading, and then it stops working
while still passing. What remains should be genuinely worth looking at.

Done with a paren-aware transform, not a regex — `var(--x, rgba(0,0,0,.5))`
nests parens and `[^)]+` would cut at the first one and leave `))` behind.
Verified after: every changed line is a fallback strip and nothing else, and
every var() reference still resolves.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UaYUaouG9jjhATyuxCKrQs

frontend/src/views/SnippetDetailView.vue

@@ -239,7 +239,7 @@ async function confirmDelete() {
 }
 .snippet-name {
   margin: 0;
-  font-family: var(--font-mono, ui-monospace, "JetBrains Mono", monospace);
+  font-family: var(--font-mono);
   font-size: 1.4rem;
   word-break: break-word;
 }
@@ -291,7 +291,7 @@ async function confirmDelete() {
 }
 .meta-grid code,
 .tag-row + * code {
-  font-family: var(--font-mono, ui-monospace, "JetBrains Mono", monospace);
+  font-family: var(--font-mono);
   font-size: 0.82rem;
   background: color-mix(in srgb, var(--color-primary) 12%, transparent);
   color: var(--color-primary);
@@ -379,7 +379,7 @@ async function confirmDelete() {
   overflow-x: auto;
 }
 .code-block code {
-  font-family: var(--font-mono, ui-monospace, "JetBrains Mono", monospace);
+  font-family: var(--font-mono);
   font-size: 0.85rem;
   line-height: 1.6;
   color: var(--color-text);