fix(#2854): slice the store-path hash from the front, not the pname suffix from the back

current_flake_rev canonicalizes to /nix/store/<hash>-<pname>; the
hash right after /nix/store/ is what varies between builds, the
trailing -<pname> is constant. slice(-12) was taking the tail, so
two different builds would very likely render the same truncated
string. slice the hash prefix out instead, with a plain head-slice
fallback for a non-store-path rev (e.g. a bare local dir in dev).

damocles caught this in review on PR #2869.

frontend/packages/dashboard/src/home.js

@@ -52,11 +52,18 @@ async function init() {
   // Rev line: absent when the flake ref isn't a local path pin (a bare
   // `github:` url has no on-disk rev to canonicalize) — hidden rather
   // than showing a placeholder in that case.
+  //
+  // `current_flake_rev` canonicalizes to a nix store path
+  // (/nix/store/<hash>-<pname>) — the part that actually varies between
+  // builds is the hash right after `/nix/store/`, not the trailing
+  // `-<pname>` (constant across revs). Slice the hash out of the front
+  // rather than truncating from the tail, so two different builds don't
+  // render as the same string. Falls back to a plain head-slice for a
+  // non-store path (e.g. a bare local dir during dev).
   const rev = $('hive-rev');
   if (rev && state.hyperhive_rev) {
-    const short = state.hyperhive_rev.length > 16
-      ? `…${state.hyperhive_rev.slice(-12)}`
-      : state.hyperhive_rev;
+    const storeMatch = state.hyperhive_rev.match(/^\/nix\/store\/([^-]+)/);
+    const short = storeMatch ? storeMatch[1].slice(0, 12) : state.hyperhive_rev.slice(0, 12);
     rev.textContent = `rev ${short}`;
     rev.title = state.hyperhive_rev;
     rev.hidden = false;