UI: tg-fullscreen header gap +10px (was +20) + font-scale regression test

The +20px gap was too far; use +10px (padding-top safe-top+16). The gap is
fixed px, so the clearance from Telegram's native nav stays constant when
the user scales the font up — the title grows downward and the bar with it,
no overflow. Locked by a new e2e test that asserts the title top is
unchanged across font sizes and never overflows the bar.

ui/src/components/Header.svelte

@@ -109,12 +109,14 @@
        back chevron is hidden), so min-height (the nav-band height) doesn't bind. Without the
        (removed) hamburger that content shrank and the bar sat flush under Telegram's native nav
        band. So **padding-top** is the lever: it drops the title clear of the band — the notch
-       plus a **+20px** gap (was +6). (Owner-tunable: bump the +20px.) */
+       plus a **10px** gap (was 6). A fixed px (not rem/em) gap so the clearance from Telegram's
+       native controls stays constant if the user scales up the font (the title then grows
+       downward and the bar with it). (Owner-tunable: the 10px.) */
     min-height: var(--tg-content-top);
     box-sizing: border-box;
     align-items: center;
     justify-content: center;
-    padding-top: calc(var(--tg-safe-top) + 26px);
+    padding-top: calc(var(--tg-safe-top) + 16px);
     padding-bottom: 6px;
   }
   :global(html.tg-fullscreen) .spacer {