Badge the installed icon, and share to the phone rather than to Downloads

Three things an installed ihasmail did not do that a phone user expects,
and all three are about the app once it is off the browser tab.

The unread count was painted into the tab title and the favicon, neither
of which exists in `display: standalone` -- so putting ihasmail on a home
screen threw the count away entirely. It goes to the Badging API as well
now. Web Push marks the icon while the app is closed, and marks it with a
dot rather than a figure: the service worker has no session to ask how
many messages are unread, and a push carries the new mail rather than a
total, so counting the payload would badge "2" over an inbox holding
forty. The next tab to open writes the real count over it.

Sharing is new. Everything that left ihasmail left as a download, which
on a phone is close to a dead end -- the file lands in Downloads and
whoever meant to send it somewhere goes looking for it in a file manager.
The share sheet is now on the message menu, on each attachment row, and
in the file viewer, which is where an attachment is already open and
where both callers meet. A message shares as text rather than as the
.eml beside it: a share sheet is aimed at everything that is not a mail
client, and an .eml in a chat app is an attachment nobody can open.

Every control feature-detects, and sharing a file is a separate question
from sharing at all -- desktop Linux and Firefox have neither, and not
every browser with `share` takes files. Anything that fails, including
the transient activation running out while a large attachment is fetched,
falls through to the download the button sits beside, so the worst case
costs a tap rather than the file. `NotAllowedError` is reported as
unsupported for that reason: it cannot be told apart from a refusal, and
a toast about activation is not something a reader can act on.

The share strings are contextual keys rather than the existing "Share…".
That one means granting another account access, and several languages use
a different verb for it -- German had "Freigeben" where the sheet wants
"Teilen". Three new strings, in all nine catalogues.

The manifest gains `launch_handler: navigate-existing`, so a mailto:, a
shortcut or a notification tapped while ihasmail is running arrives in
the copy that is running: two windows on one inbox disagree about what
has been read. `focus-existing` would have been wrong -- it only focuses
and leaves the target URL to launchQueue, which nothing here consumes, so
it would swallow the mailto. There is deliberately still no `id`, and the
manifest now says why: it is the one member resolved against the origin
of start_url rather than against the manifest's own address, so no
relative form can name a subpath mount, and the default id already is
start_url -- writing one now would give every installed copy a new
identity and orphan it as a second app.

Verified by test rather than on a device: the extension driving Chrome
was not connected, and Chrome on Linux has no Web Share to drive anyway.
The preview dialog is covered by a component test that stubs the browser
both ways.

web/public/manifest.webmanifest

@@ -3,8 +3,14 @@
   "short_name": "ihasmail",
   "description": "Fast, friendly JMAP webmail for Stalwart",
   "_comment": "JSON has no comments, so: every URL below is relative on purpose. Manifest members resolve against the manifest's own address, so these follow BASE_PATH with nothing substituted into them at build time. Root-absolute values pinned the installed app, its scope and its shortcuts to the domain root whatever the mount was.",
+  "_comment_id": "There is deliberately no `id`. It is the one member NOT resolved against this file's address -- the spec resolves it against the origin of start_url, so `./`, `mail` and `/mail` all mean the same thing at the domain root and none of them can name a subpath mount. Adding one would therefore break the same thing the note above describes. Worse, the default id IS start_url, which is already mount-correct: writing an id now would give every installed copy a new identity and orphan it as a second app rather than updating it. If one is ever wanted it has to be substituted at build time from BASE_PATH, and the changeover costs everybody their install.",
   "start_url": "mail",
   "scope": "./",
+  "categories": ["productivity", "utilities"],
+  "_comment_launch": "One window, not one per launch. A `mailto:` link, a manifest shortcut or a notification tapped while ihasmail is already open should arrive in the copy that is running rather than beside it -- two windows on the same inbox disagree about what has been read. `navigate-existing` rather than `focus-existing` because the latter only focuses and leaves the app to handle the target URL through launchQueue, which nothing here consumes: it would swallow the mailto entirely. The navigation goes through the same beforeunload guard as a reload, so an unsent draft still stops it and asks.",
+  "launch_handler": {
+    "client_mode": "navigate-existing"
+  },
   "protocol_handlers": [
     {
       "protocol": "mailto",