feat(viewer): integrate a shared SVG icon set (#1367)

* feat(viewer): integrate a shared SVG icon set

Add five Ant Design Icons (MIT) SVGs under internal/viewer/static/icons,
embed them via //go:embed, and add a CSP-safe "icon" template helper that
inlines the SVG (fill=currentColor, no script) behind a name guard.

Apply the set where the mockups show real icons: a magnifier in the
repositories search box, and the settings glyph on tool-call headers,
replacing the inline unicode gear. Accordion disclosure chevrons stay
CSS-drawn. Read-only and CSP posture are unchanged.

Closes #1321.

* feat(viewer): align the header brand logo to the mockup

Replace the hardcoded bright-green rounded-square data-URI brand-icon with
the design's monochrome aperture mark as an inline SVG (static/icons/logo.svg,
fill=currentColor) rendered via the icon helper, so the logo follows the text
color in light and dark instead of a fixed green tile. logo.svg is the
project's own brand mark; NOTICE distinguishes it from the MIT Ant Design set.

* feat(viewer): add file icon to the shared icon set

Add file.svg (Ant Design file-outlined, MIT) so the per-screen redesigns can
render a document glyph via the icon helper (e.g. the Files Reviewed list).
Not consumed yet; it completes the shared set the mockups use.

* feat(viewer): use the shared file icon in Files Reviewed

Replace the hardcoded grey ::before data-URI document glyph (which did not
follow the dark theme) with the shared inline file icon via {{icon "file"}},
so the Files Reviewed rows use the theme-adaptive icon set. Removes the old
pseudo-element rule and adds a render test.