Add descriptive page titles and social link-preview (Open Graph) metadata

[GHPBZ]

What

Gives dataset, tool/pipeline, and experiment pages their own browser title and
their own social link-preview, and replaces the site's placeholder meta
description with a real one.

• Per-dataset browser titles. The dataset page hard-coded
  <title>CONP Portal | Dataset</title> for every dataset. It now shows the
  dataset's own title (CONP Portal | {{ data.title }}), matching what the tool
  and experiment pages already do. Closes More descriptive browser page titles for the Portal #754.
• Open Graph tags for link previews. base_main.html previously shipped only
  a Twitter Card with a single site-wide title. Added standard Open Graph tags
  (og:type, og:site_name, og:title, og:description, og:image,
  og:url) so that pasting a CONP link into Slack, Teams, Bluesky, LinkedIn,
  Mastodon, iMessage, or a document produces a real preview. Dataset, pipeline,
  and experiment pages override og:title / og:description (datasets also
  og:url) so each has its own card. The Twitter Card tags now mirror the Open
  Graph values so the two never drift.
• Real meta description. <meta name="description" content="CONP"> is now a
  one-sentence description of the Portal (this is the text search engines show).
• Social handle. twitter:site is set to the CONP Bluesky handle
  @CONP_PCNO (was @NeuroLibre). Bluesky itself builds link cards from the
  Open Graph tags above; the handle tag is kept for X/Twitter attribution.

How it works

The social values are overridable Jinja blocks in base_main.html
(meta_description, og_title, og_description, og_image, og_url). Pages
that know their subject override the relevant blocks; every other page falls back
to site-wide defaults. The Twitter tags render from the same blocks via
self.og_title() etc. All per-page values have fallbacks, and dataset/tool
descriptions are passed through striptags | truncate(200) for a clean card.

Files

• app/templates/common/base_main.html — meta description; Open Graph + mirrored
  Twitter tags as overridable blocks; twitter:site set to @CONP_PCNO.
• app/templates/dataset.html — per-dataset <title>; og overrides.
• app/templates/pipeline.html — og overrides.
• app/templates/experiments/experiment.html — og overrides.

Templates only; no route, model, dependency, or build changes.

Testing

• All four templates compile under Jinja2.
• A standalone render harness confirms child block overrides flow into both the
  Open Graph and mirrored Twitter tags, that HTML in descriptions is stripped and
  escaped, and that the missing-title / missing-description fallbacks fire.

Possible follow-up

og:image is the site-wide CONP card on every page. Per-dataset images (via the
existing /dataset_logo endpoint) are a natural follow-up but were left out here
because dataset logos vary in size/availability and could produce broken cards.