feat(shared): expose live upload activity for shared files over EC (#466) - #473

This is a deliberate UI change, not a behaviour-preserving refactor --
worth being explicit about per review discussion on #675. None of the
74 sites touched here have ever rendered their border: the legacy
`Add(window, proportion, flag, border)` form only applies `border`
when `flag` carries a direction bit (wxALL/wxLEFT/wxRIGHT/wxTOP/
wxBOTTOM), and all 74 omitted it. Converting to wxSizerFlags() and
supplying a real direction bit means these borders render for the
first time, which will reflow the affected dialogs to some degree.

Per #663 (the case that originally surfaced this pattern): the
recorded border values were never validated by anything, since they
never rendered. Each site was judged against its structural siblings
rather than ported verbatim -- where siblings already carried a
working border, matched to it; where a site was the outlier in an
otherwise-consistent row/grid, adjusted to match rather than
introducing a new, never-tested value. A few sites lost their border
entirely where every sibling in the same row already had none (the
stray value read as leftover noise, not an intended margin).

Three additional non-legacy-syntax inconsistencies folded into the
same pass (found while scoping #675, confirmed still present):
- PreferencesRemoteControlsTab: "Low rights password" carried
  Border(wxLEFT|wxRIGHT, 20) while every other same-column label in
  the grid ("Web template", "Full rights password") uses
  Border(wxRIGHT, 5) -- the 20px left indent looked like it was
  copy-pasted from the unrelated UPnP-port row's indent, not a
  deliberate choice for this row.
- PreferencesOnlineSigTab: the "Save online signature file in" path
  field had no Expand()/proportion despite sitting in a column its
  parent FlexGridSizer marks growable -- it couldn't actually grow to
  fill the space reserved for it.
- PreferencesGeneralTab: the "Browser Selection" row (text field +
  Browse button) still used the legacy 3-arg Add() form while the
  structurally identical "Video Player" row already used
  wxSizerFlags() -- modernized for consistency, no behaviour change
  (both used border 0).

Scope: src/muuli_wdr.cpp only, matching where #663/#473 originally
established (and didn't fully carry through) the wxSizerFlags()
convention.

Testing: full build verified (macOS). Visually walked every dialog
these 20 functions produce that's reachable without live server/
download/client data in a fresh test config: main status bar,
search, transfer panes, shared-files header, servers/Kad tabs,
Friends/Messages panels, and all 15 Preferences tabs -- no clipped or
overlapping controls, and the two directly-testable fixes (the
OnlineSig path field now expanding, the RemoteControls password grid
column now aligned) confirmed visually.

NOT independently verified: fileDetails, clientDetails, commentLstDlg,
and CategoriesEditWindow all require live downloads/shared files/
clients/categories to reach via the UI, which a from-scratch test
config doesn't have -- these got the same siblings-based border
review as everything else, but I have not seen them rendered.
Sizer-border rendering is exactly where wxGTK/wxMSW/wxOSX diverge, so
this needs eyes on Linux and Windows too, per the #675 review
discussion.