fix(desktop): theme the table of contents text and arrows (#5094) - #5486

Closes #5094.

Reproduced

The outline panel renders the same gray on every theme. On material-dark the
label color computes to rgb(96, 98, 102) while the panel's themed color is
rgba(255, 255, 255, 0.6) — dark gray text on a dark background.

Sampling the reporter's screenshots confirms the same values: the TOC text is
#606266 in both their light and dark captures, while the sidebar file tree
text tracks the theme (#a9abae on material-dark, i.e. --sideBarColor
composited over --sideBarBgColor).

Root cause

Element Plus renders each tree node label as
<span class="el-text is-truncated el-tree-node__label">, and .el-text
declares a color of its own:

.el-text {
  --el-text-color: var(--el-text-color-regular); /* #606266 */
  color: var(--el-text-color);
}

toc.vue only sets color: var(--sideBarColor) on .side-bar-toc .el-tree, so
the label inherited the themed color — and an own color declaration always
beats an inherited one, whatever the specificity of the ancestor rule. The
expand arrow has the same problem through --el-tree-expand-icon-color
(--el-text-color-placeholder), while the sidebar file tree — a hand-rolled
component rather than an el-tree — colors its arrows with --sideBarIconColor.
That is exactly why the file tree looked right and the outline did not.

Fix

Two rules in toc.vue: let the label inherit the panel color again, and point
the expand arrow at --sideBarIconColor. Leaf arrows stay invisible — Element
hides them with visibility: hidden, not only with a transparent color.

Verification

New e2e spec test/e2e/issue-5094-toc-theming.spec.ts asserts, on both a dark
and a light theme, that the label color equals the panel's themed color, that
the arrow equals --sideBarIconColor, that neither is Element's
rgb(96, 98, 102), and that the color actually changes between the two themes.
It fails on develop with Expected: "rgba(255, 255, 255, 0.6)" / Received: "rgb(96, 98, 102)" and passes with the fix.

pnpm run lint (0 errors), pnpm run typecheck, and the TOC / theme /
layout e2e specs (17 tests) all pass.

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