fix(reactions): support max-length custom emoji (#3833)

**Category:** fix
**User Impact:** Custom emoji with valid 64-character names can now be
used as reactions without errors.

**Problem:** Buzz accepted 64-character custom emoji names during
registration, but rejected them as reactions after the required
surrounding colons made the payload 66 characters. Validation also
differed between desktop, SDK, relay, and storage boundaries.

<img width="554" height="47" alt="image"
src="https://github.com/user-attachments/assets/4013452f-210e-4dd3-9003-f45ff3b28dc8"
/>

**Solution:** Keep the product limit at 64 ASCII characters for custom
emoji names, enforce it consistently when emoji sets are registered, and
allow only valid matching custom reaction payloads up to 66 characters.
Widen the reaction projection to preserve the wrapped payload while
retaining the existing 64-character limit for ordinary reactions.

<details>
<summary>File changes</summary>

**crates/buzz-sdk/src/builders.rs**
Defines the shared custom emoji boundaries and covers accepted
64-character and rejected 65-character shortcodes.

**crates/buzz-relay/src/handlers/ingest.rs**
Validates emoji-set shortcodes and permits 66-character reactions only
when they are valid colon-wrapped custom emoji with a matching tag.

**crates/buzz-db/src/event.rs**
Adds storage regression coverage for maximum-length custom emoji
reactions.

**crates/buzz-db/src/migration.rs**
Verifies the reaction column migration is applied correctly.

**desktop/src/shared/api/customEmoji.ts**
Enforces the existing 64-character shortcode maximum during desktop
normalization and registration/import.

**desktop/src/shared/api/customEmoji.test.mjs**
Covers the desktop shortcode boundary.

**migrations/0027_long_reaction_payloads.sql**
Widens stored reaction payloads to 66 characters for the two required
surrounding colons.

**schema/schema.sql**
Keeps the desired schema aligned with the migration.

</details>

## Reproduction Steps

1. Register or import a custom emoji whose ASCII shortcode is exactly 64
characters.
2. Select that emoji as a reaction to a message.
3. Confirm the reaction publishes, persists, and renders without an
error.
4. Attempt to register a 65-character shortcode and confirm it is
rejected.
5. Publish an ordinary or malformed reaction over 64 characters and
confirm the relay rejects it.

## Verification

- `cargo test -p buzz-sdk`: 243 passed
- `cargo test -p buzz-db`: 94 passed, 152 Postgres-required tests
ignored
- `pnpm test` in `desktop`: 3,859 passed
- `cargo test -p buzz-relay`: 795 passed, 9 existing
Postgres-unavailable failures, 35 ignored; new reaction boundary tests
pass directly
- `cargo fmt --all -- --check`
- `git diff --check`

Originating Buzz channel: `f2ec9671-d78e-4cde-894c-9f4c458c7f1f`

---------

Signed-off-by: Taylor Ho <taylorkmho@gmail.com>
Co-authored-by: npub1223z34hd7vtwc6qj4s7flsxkj644nlre2nthu7lrrmkumhu3xddsrx9r6w <52a228d6edf316ec6812ac3c9fc0d696ab59fc7954d77e7be31eedcddf91335b@buzz.block.builderlab.xyz>

schema/schema.sql

@@ -539,7 +539,7 @@ CREATE TABLE reactions (
     event_created_at    TIMESTAMPTZ NOT NULL,
     event_id            BYTEA NOT NULL,
     pubkey              BYTEA NOT NULL,
-    emoji               VARCHAR(64) NOT NULL,
+    emoji               VARCHAR(66) NOT NULL,
     created_at          TIMESTAMPTZ NOT NULL DEFAULT NOW(),
     removed_at          TIMESTAMPTZ,
     reaction_event_id   BYTEA,