fix(appearance): font size setting now visibly scales UI text (closes #843)

* fix(appearance): font size setting now visibly scales UI text

Root cause: the original CSS override only changed :root{font-size} which
has no effect on the 232+ hardcoded px values throughout style.css. Only
the ~49 em/rem values were affected, which are not the main visible text.

Fix: add explicit px overrides for the key UI surfaces under each
data-font-size attribute selector:
  - .msg-body (chat messages) + headings, code, tables
  - .session-item, .session-meta (sidebar session list)
  - #msg (composer textarea)
  - .file-item (workspace file tree)

The :root override is kept so em/rem cascade correctly, but the targeted
element overrides are what actually make the text visibly larger/smaller.

Also: 8 new regression tests lock in the targeted CSS rules so this
cannot silently regress again.

* fix: composer large font was no-op — bump to 18px (default is 16px)

---------

Co-authored-by: nesquena-hermes <nesquena-hermes@users.noreply.github.com>

CHANGELOG.md

@@ -1,5 +1,15 @@
 # Hermes Web UI -- Changelog
 
+## [v0.50.147] — 2026-04-22
+
+### Fixed
+- **Font size setting now visibly changes UI text** — selecting Small or Large
+  in Appearance settings previously had no visible effect because the CSS override
+  only changed `:root{font-size}`, but the stylesheet uses 230+ hardcoded `px`
+  values that are unaffected by root font-size. Added explicit per-element overrides
+  for the key UI surfaces: chat message body, sidebar session list, composer
+  textarea, and workspace file tree. Closes #843. (#844)
+
 ## [v0.50.146] — 2026-04-22
 
 ### Fixed