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/SnippetListView.vue

@@ -574,7 +574,7 @@ function usageTitle(s: SnippetListItem): string {
   background: var(--color-bg);
   color: var(--color-text);
   font-size: 0.85rem;
-  font-family: var(--font-mono, ui-monospace, "JetBrains Mono", monospace);
+  font-family: var(--font-mono);
   box-sizing: border-box;
 }
 .loc-input-wide {
@@ -633,7 +633,7 @@ function usageTitle(s: SnippetListItem): string {
   color: var(--color-text-muted);
 }
 .empty-icon {
-  font-family: var(--font-mono, ui-monospace, "JetBrains Mono", monospace);
+  font-family: var(--font-mono);
   font-size: 2rem;
   margin-bottom: 0.75rem;
   opacity: 0.35;
@@ -720,7 +720,7 @@ function usageTitle(s: SnippetListItem): string {
   min-width: 0;
   flex: 1;
   word-break: break-word;
-  font-family: var(--font-mono, ui-monospace, "JetBrains Mono", monospace);
+  font-family: var(--font-mono);
 }
 
 /* Language tag — accent pill per the design system's tag treatment. */
@@ -770,7 +770,7 @@ function usageTitle(s: SnippetListItem): string {
   padding: 0.85rem 1rem;
   border: 1px solid var(--color-border);
   border-radius: 8px;
-  background: var(--color-surface-alt, var(--color-surface));
+  background: var(--color-surface-alt);
 }
 
 .dup-empty,
@@ -828,8 +828,8 @@ function usageTitle(s: SnippetListItem): string {
   padding: 0.1rem 0.4rem;
   border-radius: 4px;
   white-space: nowrap;
-  background: color-mix(in srgb, var(--color-danger, #b91c1c) 15%, transparent);
-  color: var(--color-danger, #b91c1c);
+  background: color-mix(in srgb, var(--color-danger) 15%, transparent);
+  color: var(--color-danger);
 }
 
 .usage-tag {
@@ -845,8 +845,8 @@ function usageTitle(s: SnippetListItem): string {
 /* Dead weight is a nudge, not an error — it warns in the warning colour rather
    than the danger one, because the record isn't broken, just unearned. */
 .usage-tag.usage-dead {
-  background: color-mix(in srgb, var(--color-warning, #b45309) 18%, transparent);
-  color: var(--color-warning, #b45309);
+  background: color-mix(in srgb, var(--color-warning) 18%, transparent);
+  color: var(--color-warning);
 }
 
 /* Header + select-mode */
@@ -906,7 +906,7 @@ function usageTitle(s: SnippetListItem): string {
 .modal-overlay {
   position: fixed;
   inset: 0;
-  background: var(--color-overlay, rgba(0, 0, 0, 0.45));
+  background: var(--color-overlay);
   display: flex;
   align-items: center;
   justify-content: center;
@@ -955,7 +955,7 @@ function usageTitle(s: SnippetListItem): string {
 .merge-choice-name {
   flex: 1;
   min-width: 0;
-  font-family: var(--font-mono, ui-monospace, "JetBrains Mono", monospace);
+  font-family: var(--font-mono);
   font-size: 0.85rem;
   word-break: break-word;
 }