style(web): tighten the in-memory caveat and punctuate the footer

Reads "Memory (this session only): Secrets are not written anywhere and
are lost on exit." — shorter tail than "lost when the Inspector exits",
and a colon separating the store from what follows it.

The colon is added at the footer's composition, not inside
`secretStorageLabel`. The label is also consumed alone by
`secretStorageSummary`, which appends " at <path>" for a file-backed
store, so a colon baked into the label would print
"File (encrypted): at /path" in the startup banner. It renders only when
a detail actually follows.

The caveat string is shared with the banner's warning line, so the
terminal picks up the shorter wording too.

Signed-off-by: cliffhall <cliff@futurescale.com>

clients/web/src/test/core/auth/secret-storage-info.test.ts

@@ -77,7 +77,7 @@ describe("secretStorageCaveat", () => {
 
   it("names the loss for memory", () => {
     expect(secretStorageCaveat(memory)).toContain(
-      "lost when the Inspector exits",
+      "lost on exit",
     );
   });
 
@@ -127,7 +127,7 @@ describe("secretStorageCaveat", () => {
     // Defensive ordering: `plaintext` is meaningless for memory, and the
     // durability loss is the more consequential of the two statements.
     expect(secretStorageCaveat({ ...memory, plaintext: true })).toContain(
-      "lost when the Inspector exits",
+      "lost on exit",
     );
   });
 });