fix(frontend): save button did nothing — arXiv ids break `#id` CSS selectors

Regression from the feed redesign. The Save button targeted
`#actions-{{ pid }}`, but every arXiv id contains a dot, and in a CSS selector
a dot starts a class token:

#actions-1706.03762 -> id="actions-1706" AND class="03762"

A class token also cannot begin with a digit, so this is not a near miss —
querySelector raises SelectorSyntaxError ("Malformed class selector at
position 13", confirmed against a real CSS engine). htmx could not resolve the
target, so the swap never ran and clicking Save appeared to do nothing.

The POST itself was always fine; only the UI never updated.

Restores the attribute-selector form the pre-redesign code used deliberately,
now with a comment explaining why so it does not regress again. Same bug and
same fix in the onboarding seed picker, which was silently broken for every
seed save.

Verified: 12/12 save targets in a live feed resolve to exactly one element;
save round-trip returns the Saved state, keeps "Why this?", and writes the
interaction row.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app/templates/partials/action_buttons.html

@@ -60,9 +60,17 @@
           data-paper-id="{{ pid }}"
           data-vals='{{ _vals | tojson }}'>Remove</button>
 {% else %}
+  {#
+    The target MUST be the attribute-selector form, not "#actions-{{ pid }}".
+    Every arXiv id contains a dot (1706.03762), and in a CSS selector that dot
+    starts a class token — so "#actions-1706.03762" parses as
+    id="actions-1706" AND class="03762", which matches nothing. A class token
+    also cannot begin with a digit, so querySelector raises a SyntaxError
+    rather than merely missing. [id='…'] compares the literal string.
+  #}
   <button type="button" class="btn btn-sm btn-save"
           hx-post="/api/papers/{{ pid }}/save"
-          hx-target="#actions-{{ pid }}"
+          hx-target="[id='actions-{{ pid }}']"
           hx-swap="innerHTML"
           hx-vals='{{ _vals | tojson }}'>
     <svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.9"

app/templates/partials/seed_results.html

@@ -18,7 +18,9 @@
         </div>
         <button type="button" class="btn btn-sm btn-save"
                 hx-post="/api/papers/{{ paper.arxiv_id }}/save"
-                hx-target="#seed-paper-{{ paper.arxiv_id }}"
+                {# Attribute selector, not "#seed-paper-…": the dot in an arXiv
+                   id would be read as a CSS class token. See action_buttons.html. #}
+                hx-target="[id='seed-paper-{{ paper.arxiv_id }}']"
                 hx-swap="outerHTML"
                 hx-vals='{"source": "onboarding", "position": {{ loop.index0 }}}'
                 onclick="if (window.bumpSeedCount) window.bumpSeedCount()">Save</button>