web: empty states and contribution hints sweep #270

Web empty states and contribution hints: CLI instructions for web visitors, inconsistent copy.

• mrs.html:29 tells logged-out visitors "no open merge requests — open one with gitbay mr create krz/gitbay --source ... --target main", and under ?state=merged reads "no merged merge requests — open one with ...".
• mr.html:136,143,174 shows "none yet" / "nobody yet" on merged and closed MRs.
• Empty-state wording differs in case and form: "no builds — push a commit with a .gitbay/ci.yml", "Nothing pinned yet. Press Pin on a repository.", "nothing unread — show all", "nothing here yet".
• A reader without write access sees no path to contribute: "New merge request" shows only to signed-in viewers, the fork hint is one sentence after the form in mrnew.html, and the Fork button needs a session.
• Global search explains what it covers only before the first query (globalsearch.html:47).
• Repository tabs read "Issues 9", then "Merge requests" with no count.

Changes:

• One empty-state register across templates, as one sweep. No "yet" on a finished item. A create link only when the viewer can create; never a CLI command on the web.
• MR list: "Fork this repository to propose a change" for a viewer who cannot push; a sign-in prompt for a signed-out one.
• Search: the scope sentence as a permanent caption under the input.
• Tabs: one rule for zero counts.