feat(web): row-count selector on the Executions card

Adds a "Show" selector (10 / 25 / 50 / 100) to the card header and drops the
default row count from 15 to 10. The chosen value rides on the card's own
polling URL (`/executions?limit=N`), so it survives the 5s htmx refresh with no
client-side state; a config default outside those four is rendered as an extra
option instead of disappearing. The query is parsed permissively — a stale or
hand-edited polling URL clamps or falls back to the config default rather than
422-ing a card that would otherwise render.

Ordering is now explicit rather than inherited from whatever Gitea returns:
newest first, a not-yet-started (queued) run above everything, ties broken on
Gitea's run id so matrix runs sharing a timestamp keep a stable order.

config.example.toml

@@ -103,7 +103,7 @@ retention_days = 30
 # only -- so neither key below is required. Set them to cover the cases that
 # token cannot: a service restart (which forgets the logged-in tokens) or a
 # non-admin operator.
-# executions_limit    = 15                 # how many runs to list (1..100)
+# executions_limit    = 10                 # default rows in the card's selector (1..100)
 # gitea_token_cred    = "GiteaApiToken"    # keyring KEY NAME of a fallback PAT
 #
 # Gitea OIDC client registration: Gitea -> Settings -> Applications -> OAuth2,