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.

FEATURES.md

@@ -1141,6 +1141,32 @@ needed nothing in either half.
   installability and fast loads, API requests never are, and navigations are
   network-first with the shell as fallback.
 - **Manifest shortcuts** for Compose, Calendar and Contacts.
+- **One window, not one per launch.** A `mailto:` link, a shortcut or a
+  notification opened while ihasmail is already running arrives in the copy
+  that is running. Two windows on the same inbox disagree about what has been
+  read, and only one of them is where the half-written reply is.
+- **The unread count on the installed app's icon.** The tab title and the
+  painted favicon are the same idea for a browser tab, and an installed app has
+  neither -- in `display: standalone` there is no tab strip and no favicon on
+  screen, so a home-screen ihasmail showed nothing at all. Web Push marks the
+  icon while the app is closed, 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.
+  Unsupported browsers show nothing, as does iOS until notification permission
+  has been granted, which is that platform's condition for a badge.
+- **Share** — a message, or one attachment, handed to the operating system's
+  share sheet instead of to the filesystem. On a phone a download is close to a
+  dead end: the file lands in Downloads and whoever wanted to send it somewhere
+  goes hunting for it in a file manager. The sheet is on the message menu, on
+  each attachment row, and in the file viewer, which is where an attachment is
+  already open. A message shares as text rather than as the `.eml` beside it,
+  because 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 one of those
+  controls is drawn only where the browser has Web Share -- absent on desktop
+  Linux and in Firefox -- and sharing a file is asked about separately from
+  sharing at all. Where the share cannot be made, the download it sits beside
+  happens instead, so the worst case costs a tap rather than the file.
 - **`mailto:` handler** — registered from Settings › General for the browser
   (needs HTTPS; Safari does not support it), and declared in the manifest so an
   installed ihasmail is offered by the operating system wherever something asks