fix: keep a pct-encoded UTF-8 sequence whole, and bound the preview too

Addresses Copilot's round-16 review on #2035. Two suppressed comments, both
real, and both refinements of fixes from the previous two rounds rather than
defects in the original code.

1. Round 11 settled on "a pct-encoded triplet counts as one character", but
   a multi-octet character spans several triplets -- `%C3%A9` is one `é`.
   Measured on a3dd8563:

     x{+v:1}  v = "%C3%A9x"  =>  "x%C3"   a lone lead byte, decoding to nothing

   `prefixUnits` now recognizes a well-formed pct-encoded UTF-8 sequence from
   its lead byte (RFC 3629: C2-DF two octets, E0-EF three, F0-F4 four;
   continuations 80-BF), longest alternative first so a four-octet sequence
   is not read as a two-octet one followed by loose triplets.

   A malformed sequence -- a lead byte with too few continuations, or a stray
   continuation -- matches no sequence alternative and falls back to
   per-triplet counting. That is the conservative direction: it counts as
   MORE units, so truncation keeps less rather than emitting octets that
   were never in the value.

2. Round 14's per-value ceiling only covered the read. The panel calls
   `previewUriTemplate` on every keystroke, and that path expands expression
   by expression, so it reached the allocation-heavy encoders with the same
   oversized value -- submission refused while the UI froze on the paste
   anyway. Both paths now share one exported `valueLengthError`, checked
   before any encoding happens.

npm run ci passes.

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

clients/web/src/utils/uriTemplate.test.ts

@@ -126,3 +126,19 @@ describe("previewUriTemplate - it must never throw or over-promise", () => {
     );
   });
 });
+
+describe("previewUriTemplate - the value ceiling", () => {
+  it("falls back before encoding an oversized value", () => {
+    // Without this the guard was half a guard: the read was refused while the
+    // preview still encoded the same value on every keystroke, so a large
+    // paste froze the UI anyway.
+    expect(previewUriTemplate("x://{v}", { v: "a".repeat(1_000_001) })).toBe(
+      "x://{v}",
+    );
+  });
+
+  it("still previews a value at the limit", () => {
+    const value = "a".repeat(1_000_000);
+    expect(previewUriTemplate("x://{v}", { v: value })).toBe(`x://${value}`);
+  });
+});