Allow a notification keyword to start with a dot (#34574)

Both notification settings tabs stored a keyword under a push rule whose id
was the keyword itself. Homeservers reserve the rule ids beginning with a
dot for the rules they define, and Synapse refuses to create any other, so
saving a keyword such as "...push complete" failed outright and the whole
save was reported as an error.

The rule id is an internal name that the user never sees — both tabs list
keywords by the rule's pattern — so it is the id that gives way. It now
drops the leading dots while the pattern keeps the keyword exactly as it
was typed, which is what is matched against messages. Two keywords can
want the same id that way, so a number is appended when one is taken,
which also keeps "banana" from overwriting the rule for ".banana".

Tests: the shared reconciler and the older tab both store a dotted keyword
under an accepted id, and two keywords differing only by a leading dot get
an id each.

apps/web/test/unit-tests/components/views/settings/Notifications-test.tsx

@@ -858,6 +858,20 @@ describe("<Notifications />", () => {
             });
         });
 
+        it("adds a keyword that starts with a dot", async () => {
+            await getComponentAndWait();
+
+            await userEvent.type(screen.getByLabelText("Keyword"), ".jest");
+
+            fireEvent.click(screen.getByText("Add"));
+
+            // The homeserver rejects a rule id beginning with a dot, so only the pattern keeps it.
+            expect(mockClient.addPushRule).toHaveBeenCalledWith("global", PushRuleKind.ContentSpecific, "jest", {
+                actions: [PushRuleActionName.Notify, { set_tweak: "highlight", value: false }],
+                pattern: ".jest",
+            });
+        });
+
         it("adds a new keyword with same actions as existing rules when keywords rule is off", async () => {
             const offContentRule = {
                 ...bananaRule,