Rebrand to INBUXA: product name, protocol greetings, sign-in page, calendar templates, logos, README

One branding module (types::brand!) supplies the name to every protocol
greeting, the HTTP realm, the startup banner, Received headers, the
user agent, the IMAP ID response and calendar PRODIDs. The sign-in and
calendar pages take ihasmail's palette and font and the INBUXA logo, and
calendar emails embed the INBUXA lockup. Default calendar and address book
names follow. Protocol identifiers (urn:stalwart:jmap, vnd.stalwart Sieve
extensions) and upstream copyright notices are unchanged.

img/brand/README.txt

@@ -0,0 +1,28 @@
+inbuxa logo bundle
+===================
+
+Mark: reused from ihasmail's actual cat-and-envelope artwork (ihasmail-mark.png),
+unmodified. Colors sampled directly from that file:
+  navy outline   #1C4053
+  cat orange     #F9A34C  (inner ear #F47F35)
+  envelope teal  #46CAC3
+
+Wordmark set in Space Grotesk (Bold for "inbuxa", Medium for the "MAIL SERVER"
+tag line), rendered as true vector paths in the SVGs -- no font install needed
+to view or edit them correctly in any vector app.
+
+Files
+-----
+inbuxa-icon.svg / .png              Mark only, square, for favicons/app icons
+inbuxa-lockup-light.svg / .png      Full hero lockup, white background
+inbuxa-lockup-dark.svg / .png       Full hero lockup, dark background
+inbuxa-lockup-compact-light.svg/.png  Compact lockup (icon + "inbuxa" only),
+                                       built at exactly 5x a 190x40 target
+inbuxa-lockup-compact-dark.svg/.png   Same, dark background
+inbuxa-lockup-190x40-light.png      Compact lockup pre-rendered at the literal
+                                     190x40 target size
+inbuxa-lockup-190x40-dark.png       Same, dark background
+
+Dark backgrounds use #0B1720 rather than pure black -- a shade darker than the
+mark's own navy outline, so the outline still reads as a lighter edge instead
+of disappearing into the page.