Give every fastext destination a distinct mark; add skip-link, fix aria-label

Colours: "if the intent is to have them all different colors than
lets have them all different colors." The locked eight only give five
safe as full text (blue fails contrast, black's invisible on black),
so the other three get their own treatments instead of sharing plain
ink: Watch gets blue as a swatch-only accent (the button cap's colour,
not the label), Activity gets an ordered-dither swatch (the other
legal way to mark something without a ninth colour), and Settings
alone keeps plain ink deliberately, as the one page with no live
status of its own.

Accessibility (items 1 and 3 from the design review, item 2 skipped
per request — the header's download figure stays live and unmuted):
- Added a skip-link, off-screen until focused, jumping past the
  8-item top nav straight to each page's #main-content.
- Renamed the nav's aria-label from "Fastext" (meaningless without
  the visual teletext context) to "Page navigation".

CLAUDE.md

@@ -105,18 +105,22 @@ the user's Unraid home server. Server-side project directory is still named
     under the header on every page but the index — those pages carry no
     other nav, so it needs no scroll to reach. The index overrides it
     with `HOME_FASTEXT`, the original four-entry row, as its own closing
-    line, since its body already lists every destination once. The
-    original four (Index/Library/Discover/System) keep their `r`/`g`/
-    `y`/`c` keyboard shortcut and colour; Requests also gets a colour
-    (magenta, `--pending`) without a shortcut — a deliberate fit, not
-    an arbitrary fifth pick, since Requests already is "waiting on a
-    human" everywhere else magenta appears. That's the ceiling: the
-    locked eight colours leave exactly five that read as legible text
-    on black (blue fails contrast there, black is invisible), so Watch/
-    Activity/Settings share plain ink rather than a sixth colour that
-    doesn't exist in this palette. Whichever entry matches the current
-    page renders as a `.fastext-here` span instead of a link — a page
-    linking to itself did nothing, so it isn't offered as a click.
+    line, since its body already lists every destination once. Every
+    destination gets its own distinct mark — the locked eight colours
+    only give five that read as legible text on black, so getting to
+    eight took three treatments: five flat colours as text+swatch
+    (Index/Library/Discover/System keep their `r`/`g`/`y`/`c` shortcut;
+    Requests gets magenta/`--pending` with no shortcut — a deliberate
+    fit, since Requests already is "waiting on a human" everywhere else
+    magenta appears); Watch gets blue as a swatch-only accent (`.k-blue`
+    — blue fails contrast as body text, so only the small cell carries
+    it, not the row's own label); Activity gets an ordered-dither swatch
+    (`.k-dither`, same mechanism as `--dither-mid`) rather than a flat
+    colour. Settings alone keeps plain ink with no swatch treatment —
+    deliberately, as the one page with no live status of its own.
+    Whichever entry matches the current page renders as a
+    `.fastext-here` span instead of a link — a page linking to itself
+    did nothing, so it isn't offered as a click.
   - **Nothing is clipped, hidden, or sticky.** All seven page numbers and
     every fastext entry stay visible at every width, reflowing to another
     row rather than scrolling, truncating, or folding into a menu. No