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/KnowledgeView.vue

@@ -490,7 +490,7 @@ onUnmounted(() => {
 .knowledge-root {
   display: flex;
   flex-direction: column;
-  height: calc(100vh - var(--header-height, 56px));
+  height: calc(100vh - var(--header-height));
   overflow: hidden;
 }
 
@@ -502,7 +502,7 @@ onUnmounted(() => {
   gap: 12px;
   padding: 8px 20px;
   background: var(--color-bg-secondary);
-  border-bottom: 1px solid var(--color-border, rgba(255,255,255,0.06));
+  border-bottom: 1px solid var(--color-border);
   flex-shrink: 0;
   font-size: 0.82rem;
   flex-wrap: wrap;
@@ -539,7 +539,7 @@ onUnmounted(() => {
   width: var(--sidebar-width);
   flex-shrink: 0;
   padding: 16px 12px;
-  border-right: 1px solid var(--color-border, rgba(255,255,255,0.06));
+  border-right: 1px solid var(--color-border);
   overflow-y: auto;
   background: var(--color-bg-secondary);
 }
@@ -683,7 +683,7 @@ onUnmounted(() => {
   gap: 10px;
   padding: 12px 20px;
   flex-shrink: 0;
-  border-bottom: 1px solid var(--color-border, rgba(255,255,255,0.06));
+  border-bottom: 1px solid var(--color-border);
 }
 .search-wrap {
   flex: 1;
@@ -701,8 +701,8 @@ onUnmounted(() => {
   width: 100%;
   padding: 7px 12px 7px 32px;
   border-radius: 8px;
-  border: 1px solid var(--color-border, rgba(255,255,255,0.1));
-  background: var(--color-bg-tertiary, rgba(255,255,255,0.04));
+  border: 1px solid var(--color-border);
+  background: var(--color-bg-tertiary);
   color: var(--color-text);
   font-size: 0.88rem;
   outline: none;
@@ -712,8 +712,8 @@ onUnmounted(() => {
 .sort-select {
   padding: 7px 10px;
   border-radius: 8px;
-  border: 1px solid var(--color-border, rgba(255,255,255,0.1));
-  background: var(--color-bg-tertiary, rgba(255,255,255,0.04));
+  border: 1px solid var(--color-border);
+  background: var(--color-bg-tertiary);
   color: var(--color-text);
   font-size: 0.85rem;
   cursor: pointer;
@@ -735,9 +735,9 @@ onUnmounted(() => {
 
 .k-card {
   position: relative;
-  background: var(--color-surface, rgba(255,255,255,0.03));
-  border: 1px solid var(--color-border, rgba(255,255,255,0.07));
-  border-radius: var(--radius-lg, 14px);
+  background: var(--color-surface);
+  border: 1px solid var(--color-border);
+  border-radius: var(--radius-lg);
   padding: 14px;
   cursor: pointer;
   transition: border-color 0.15s, transform 0.12s, box-shadow 0.15s;
@@ -917,7 +917,7 @@ onUnmounted(() => {
 .graph-panel {
   width: 500px;
   flex-shrink: 0;
-  border-left: 1px solid var(--color-border, rgba(255,255,255,0.06));
+  border-left: 1px solid var(--color-border);
   display: flex;
   flex-direction: column;
   background: var(--color-bg-secondary);
@@ -933,7 +933,7 @@ onUnmounted(() => {
   padding: 10px 14px;
   font-size: 0.85rem;
   font-weight: 500;
-  border-bottom: 1px solid var(--color-border, rgba(255,255,255,0.06));
+  border-bottom: 1px solid var(--color-border);
   flex-shrink: 0;
 }
 /* Override GraphView's 100vh height so it fills the panel instead */