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/components/HistoryPanel.vue

@@ -403,12 +403,12 @@ onMounted(loadVersions);
   font-size: 0.85em;
   line-height: 1;
 }
-.pin-badge-manual { color: var(--color-primary, #6366f1); }
-.pin-badge-auto { color: var(--color-text-muted, rgba(255, 255, 255, 0.5)); }
+.pin-badge-manual { color: var(--color-primary); }
+.pin-badge-auto { color: var(--color-text-muted); }
 
 .history-item-label {
   font-size: 0.72rem;
-  color: var(--color-primary, #6366f1);
+  color: var(--color-primary);
   font-style: italic;
   margin-top: 0.15rem;
   overflow: hidden;
@@ -430,7 +430,7 @@ onMounted(loadVersions);
 }
 .pin-state {
   font-style: italic;
-  color: var(--color-text-muted, rgba(255, 255, 255, 0.6));
+  color: var(--color-text-muted);
   flex: 1;
   min-width: 0;
   overflow: hidden;
@@ -442,13 +442,13 @@ onMounted(loadVersions);
   font-size: 0.78rem;
   background: transparent;
   color: inherit;
-  border: 1px solid var(--color-border, rgba(255, 255, 255, 0.12));
+  border: 1px solid var(--color-border);
   border-radius: 999px;
   cursor: pointer;
 }
 .btn-pin:hover:not(:disabled), .btn-pin-edit:hover:not(:disabled) {
   background: rgba(99, 102, 241, 0.12);
-  border-color: var(--color-primary, #6366f1);
+  border-color: var(--color-primary);
 }
 .btn-unpin:hover:not(:disabled) {
   background: rgba(239, 68, 68, 0.10);
@@ -463,27 +463,27 @@ onMounted(loadVersions);
   flex: 1;
   padding: 0.3rem 0.5rem;
   font-size: 0.85rem;
-  background: var(--color-input-bg, rgba(255, 255, 255, 0.03));
-  border: 1px solid var(--color-border, rgba(255, 255, 255, 0.12));
-  border-radius: var(--radius-sm, 4px);
+  background: var(--color-input-bg);
+  border: 1px solid var(--color-border);
+  border-radius: var(--radius-sm);
   color: inherit;
 }
 .pin-label-input:focus {
   outline: none;
-  border-color: var(--color-primary, #6366f1);
+  border-color: var(--color-primary);
 }
 .btn-pin-save, .btn-pin-cancel {
   padding: 0.3rem 0.7rem;
   font-size: 0.78rem;
   background: transparent;
   color: inherit;
-  border: 1px solid var(--color-border, rgba(255, 255, 255, 0.12));
-  border-radius: var(--radius-sm, 4px);
+  border: 1px solid var(--color-border);
+  border-radius: var(--radius-sm);
   cursor: pointer;
 }
 .btn-pin-save:hover:not(:disabled) {
   background: rgba(99, 102, 241, 0.12);
-  border-color: var(--color-primary, #6366f1);
+  border-color: var(--color-primary);
 }
 .btn-pin-save:disabled, .btn-pin-cancel:disabled,
 .btn-pin:disabled, .btn-pin-edit:disabled, .btn-unpin:disabled {